The Embryology and Affinities of the Symphyla, based on a Study of Hanseniella agilis
Authors: Tiegs O. W. 
Published: April 2021

The egg is heavily yolked and devoid of periplasm. Shortly before laying, the large germinal vesicle disrupts, only a minute clump of chromatin surviving. At laying, the egg is in the first meiotic prophase. The polar bodies remain part of the egg and degenerate rapidly. Nuclear fusion takes place in the centre of the egg. Cleavage is total, and there is a cleavage-cavity. At about the one-hundred-cell stage the irregularly shaped yolk-pyramids begin to undergo tangential division, yielding an outer layer of smaller and an inner layer of larger cells. Both are rich in yolk. The outer layer forms eventually the blastoderm, and from the latter is secreted a blastodermic cuticle ; the inner, by repeated division of its cells, yields a solid mass of ‘yolk-cells’ in which cell-walls soon disappear. A large proportion of the ‘yolk-cells’ later degenerates; the remainder are utilized in the formation of fat-body and mid-gut epithelium. A remarkable ‘dorsal organ’ develops. Above it there is no blastodermic cuticle. From each cell of the organ arises a long tendril which grows in the space between the chorion and blastodermic cuticle on to the opposite pole of the egg. In the advanced embryo the organ degenerates ; the ‘tendrils ‘remain with the chorion at eclosion. The blastoderm soon differentiates into an embryofac region and a thin membrana dorsalis (provisional body-wall). A precocious flexure develops in the embryonic region, as in some diplopods, by the formation of a pair of lateral clefts which cut into the yolk and transect the lower two-thirds of the egg. This gives form to the germ-band. Within the germ-band there soon becomes distinguishable a head, in which there is no external sign of segmentation, followed by a labial segment which is at first part of the abdomen rather than of the head. The remaining abdominal segments develop in succession from before backwards. The posterior end of the germ-band is a region of growth, and as new segments arise there, the more anterior ones become closely compressed with the formation of deep intersegmental clefts. The labial segment merges into the head. In the abdomen there are seven segments and an ‘end-piece’, the latter segmenting later into a pre-anal and an anal segment. On the fourth and sixth segments two tergal scutes develop; the remaining segments are provided each with a single scute. An embryonic cuticle develops, devoid of setae but furnished with two pairs of cutting-hooks on the head; the larval cuticle develops below it. The embryonic cuticle is shed at eclosion. The abdominal appendages develop as blunt outgrowths along the ventro-lateral margin of the germ-band. The first seven segments form each a single pair of legs. The appendage of the pre-anal segment becomes the cercus. The diminutive appendage of the anal segment forms the trichobothrium. Additional legs arise during anamorphosis from the pre-anal segment ; the first of these (eighth leg) is in an advanced state of development at eclosion. The legs present evidence of not more than five segments in the embryo; the limb-base (subcoxa ?) is never part of the leg. The head presents evidence of six segments, as in other myriapods and insects. The conversion of the embryonic segments into a compact head is attended by much migration of the sternal ectoderm which, despite the absence of intersegmental lines, can be followed by the aid of the ‘ventral organs The antenna becomes pre-oral and much of the front of the head is antennal in origin. The pre-mandibular ectoderm curves round the stomodaeal opening, and forms much of the inferior surface of the clypeo-labrum. The floor of the pre-oral cavity arises from the sterna of the pre-mandibular and gnathal segments, as in insects. In addition to the antennary, mandibular, maxillary, and labial appendages there is present, in the embryo, a pair of very transient pre-mandibular appendages, but no pre-antennary appendages. The superlinguae develop from the ‘ventral organs’ of the mandibular segment, and are therefore not appendages. Both maxillary and labial ectoderm enter into the formation of the hypopharynx. The mesoderm arises by ingrowth of cells from the walls of the two lateral clefts. It accumulates mainly along the lateral margin of the germ-band, where it becomes segmented into two rows of somites, with an intervening thin band of median unsegmented mesoderm. Somites appear first in the three gnathal segments, thence spreading forward in the head, and backwards along the abdomen. There are six pairs in the head and a single pair in each abdominal segment. In each somite a coelomic cavity develops, that of the pre-antennary segment being diminutive, those of the pre-mandibular and anal segments rather small, but the others widely expanded. Most of the coelomic cavities communicate by fine intersegmental channels. At the height of their development the abdominal coelomic sacs are trilobed vesicles, each with a dorso-lateral, a medio-ventral, and an appendicular lobe. In the gnathal segments the medio-ventral lobes are absent. From the wall of the pre-anal coelomic sac that of the eighth (i.e. first post-embryonic) abdominal segment develops. From the pre-mandibular coelomic sacs develop a pair of remarkable ‘excretory glands’ which disrupt at about the time of eclosion. In association with them arise, also from the pre-mandibular mesoderm, clumps of ‘brown cells’ which are carried back into the beginning of the abdomen and survive as the nephrocytic organs. They are probably homologous with the sub-oesophageal lymphoid tissue of insects. The salivary glands are remnants of the maxillary coelomic sacs. The spinning-glands of the cercus are derived from the pre-anal mesoderm, but their cavities are not the cavities of the original coelomic sacs. The ‘paracardial glands’ are derived from the mandibular mesoderm. The abdominal coelomic sacs are attached to the overlying epidermis at the intersegments. During the eighth day they begin to grow through the yolk-cells, some of which become enclosed as an axial column, from which will arise the mid-gut ; from the remainder will form the fat-body. By close apposition of visceral and somatic walls the original coelomic cavities become almost obliterated ; the only vestiges are a pair of cavities to the sides of the developing heart (pericardial cavity) and a pair of genital tubes to the sides of the developing mid-gut. The visceral walls now become detached from the somatic walls and give rise to the splanchnic muscle. All the coelomic sacs from the mandibular to the pre-anal contribute splanchnic mesoderm to the mid-gut. The heart arises by enclosure of a tube between two rows of cardioblasts, derived from cells located in the dorso-median walls of the original coelomic sacs. All the coelomic sacs from the mandibular to anal are involved. The anal coelomic sacs survive into the larva and seem to furnish the cells from which the hinder end of the heart will develop during anamorphosis. The pericardial septum is formed from the somatic walls of the coelomic sacs, which remain in position after the visceral walls have become drawn up on to the mid-gut, and which have now become converted into extremely tenuous membranes. Only the posterior attachment at the intersegmental epidermis survives. The pericardial septum therefore forms a broken partition, with wide fan-shaped expansions on the heart-wall. In the first three segments the septum is disrupted owing to withdrawal of the mid-gut, to which it is at the time attached, from that region of the embryo. The aorta arises mainly from antennary mesoderm, but the anterior blood-distributing funnel is derived from the pre-antennary mesoderm. The antennary and cephalic arteries are formed out of narrow cords of vasoblasts derived respectively from the antennary and mandibular mesoderm. The ventral blood-vessel and median sternal artery arise from cells of the median mesoderm. Formation of the haemocoele begins with the appearance of an epineural sinus, due to shrinkage of the yolk from the underlying ectoderm. The greater part of the definitive bloodspace is not apparent till after eclosion. The genital tubes are the vestiges of the medio-ventral lobes of the two rows of original coelomic sacs that have survived near the ventro-lateral walls of the developing midgut. Until shortly before eclosion they retain their connexion with the pericardial septum. The anterior limits of the tubes are originally in the first abdominal segment; but with the breakdown of the pericardial septum in the first three abdominal segments the anterior limit of the genital tube comes to lie in the fourth abdominal segment. From most of the abdominal coelomic sacs rudimentary coelomoducts develop, only the pair from the pre-anal segment apparently surviving into the larva. The first instar larva presents the appearance of a potentially opisthogoneate myriapod. The secondary genital opening on the fourth segment develops in a later larval instar. The germ-cells are very few in number and are first recognizable in the medio-ventral walls of certain of the hinder abdominal coelomic sacs. The mid-gut epithelium develops out of the axial column of ‘yolk-cells’ which have become enclosed within splanchnic mesoderm ; a lumen does not appear between the mid-gut cells till in the advanced embryo. Stomodaeum and proctodaeum arise as invaginations of the ectoderm. Stomodaeal muscle is formed mainly from the antennary mesoderm, proctodaeal muscle from the mesoderm of the anal and pre-anal segments. The two malpighian tubes are derived from the tip of the proctodaeum. The ventral nerve-cord develops as a pair of lateral cords of ectoderm cells which move closer to the mid-line and become associated with a median cord. The latter contributes to the formation of the ganglia. There is a single ganglion in each segment, the heaping up of the ganglion-cells in an anterior and a posterior mass in most of the segments giving the false impression of a duplication of ganglia as in diplopods. The most noteworthy feature of the developing ganglia is their association with ‘ventral organs’ of the kind found in Peripatus. In later embryos these dwindle in size and survive as the eversible sacs. In the mandibular segment they become the superlinguae. Ventral organs develop from the antennary to the pre-anal segments. The brain develops out of (a) a pair of large trilobed protocerebral ganglia, (b) a pair of diminutive pre-antennary ganglia (unknown in insects), (c) a pair of antennary ganglia, (d) a pair of pre-mandibular ganglia. The median cord does not enter into the formation of the brain. The protocerebral ganglia arise from the roof of the head (acron?) and are not serially homologous with the other three component ganglia of the brain. The neurilemma, both of the brain and of the ventral nervecord, is derived from mesoderm. The stomatogastric ganglia are developed from the stomodaeum. The organs of Tômôsvary arise from the ectoderm in close association with the lateral lobes of the protocerebrum. The tracheae arise in the late embryo as a pair of ectodermal invaginations just in front of the mandibles. They do not function till the second larval instar. The muscles of the appendages develop out of the cells of the appendicular lobes of the coelomic sacs. The dorsolateral body-muscles arise from cells that pass with the dorsolateral lobes of the coelomic sacs on to the median dorsal surface of the embryo. The other abdominal muscles, including the ventral longitudinal muscles, arise from clumps of cells at the bases of the appendages, derived from the original coelomic sacs. While precluded, through their progoneate condition, from the direct line of ancestry of insects, the Symphyla may be regarded as the remnants of an ancient stock of myriapods, closely related to the opisthogoneate ancestors of insects, but already characterized by the formation of a secondary genital opening, which is one of the distinctive features of the pauropods and diplopods.