The Embryonic Development of Calandra oryzae
Authors: Tiegs O. W. , Murray Florence V. 
Published: April 2021

In the maturation of the egg, although post-reduction appears to occur, there is actually pre-reduction, but much obscured owing to a separation of precociously split chromosomes late in the first meiotic anaphase. A temporary separation of conjugated chromosomes also precedes the first meiotic division. Cleavage (non-synchronized) follows rapidly upon fusion of male and female pro-nuclei. The cleavage-cells spread through the yolk, apparently by their own activity. A cleavage pattern, though not directly observable, is to be inferred on theoretical grounds. The cleavage-cells become drawn into the periplasm by a centrifugal flow of the cytoplasm. Upon entering the periplasm, or just before this, the nuclei divide. Early cleavages in the blastoderm are therefore synchronized. Later the synchronization disappears, though other remarkable forms of co-ordination have been encountered. As the blastoderm matures, the cells, now much diminished in size, develop first lateral and then inner cell-walls, the latter within the secondary periplasm. The yolk-cells are derived from a small number of cells that do not enter the periplasm. They divide apparently solely by mitosis. The blastoderm does not contribute appreciably to their number. The germ-cells are part of the blastoderm, and protrude prominently at the hinder end. Later they become withdrawn level with the blastoderm. They early become infected with bacteria from a large bacterial mass in the adjacent yolk. The mycetocytes of the ovary arise at this time by migration of adjacent blastoderm-cells into the bacterial mass. The germ-band arises, as usual, by a dorsal thinning and a ventral and lateral thickening of the blastoderm. Beginning at the anterior end the median and two lateral plates become demarcated, the latter then invaginating with formation of a temporary gastral groove. The invaginated cells form the inner layer, which is entirely mesodermal. The germ-band, meanwhile, grows over the hinder pole of the egg and extends to the anterior end, carrying the germcells with it. Associated with this is a peculiar method of amnion-formation due to deep invagination of the germ-band into the yolk; on the ventral surface of the egg the amnion arises by downgrowth of folds along the margin of the germ-band. Segmentation of the germ-band proceeds from before backwards, without the formation of macrosegments. A vanished twelfth segment is inferred for the abdomen. The appendages develop approximately in order from before backwards. The labrum appears later. In the abdomen there are no appendages. Shortening of the germ-band occurs on the third day, and thereafter the larval form is gradually assumed, the lateral body-wall growing upwards over the yolk. The formation of the head is described in detail. The thoracic appendages merge into the body-wall in the advanced embryo, and form, then, the imaginai disks of the legs. Amnion and serosa do not rupture, but form a permanent enclosure for the embryo. The stomodaeum marks the anterior limit of the invaginated inner layer, the pre-oral mesoderm arising by the spreading forward of cells from behind the stomodaeum. The post-oral mesoderm differentiates into a median unsegmented sheet of cells and two lateral rows of somites, extending from the labial to the tenth abdominal segment ; elsewhere the mesoderm remains unsegmented. With the exception of the tenth abdominal, these somites expand into coelomic sacs. A coelomic sac forms later in the antennary segment also. On the third day, the cavities of the coelomic sacs having become confluent, differentiation proceeds: the splanchnic wall forms the splanchnic muscle of the gut ; the inferior wall becomes the fat-body; the dorso-lateral wall forms the heart tissue, while the lateral wall becomes resolved into the lateral plate myoblasts, from which much of the somatic musculature develops. The remaining somatic muscles are derived from the subsomitic mesoderm, into which is incorporated most of the median unsegmented mesoderm. With the withdrawal of the median mesoderm the epineural sinus is formed between the yolk and the nerve-cord; by its enlargement the haemocoele is developed. The stomodaeum and proctodaeum arise as simple ingrowths of the outer layer, the latter occurring at the posterior limit of the germ-band. The mid-gut has a bipolar origin. It forms from the blind ends of the stomodaeum and proctodaeum, quite independently of the inner layer. The mid-gut ‘Anlagen’ grow towards one another and meet in the first or second abdominal segment. They eventually wholly enclose the yolk. The difficulty of reconciling such facts with the germlayer theory is discussed. The malpighian tubes arise from the proctodaeum, the first pair much preceding the other two in time of development. The remarkable adaptation of the development of the intestine to the bacterial symbiont is described. The sub-oesophageal bodies arise from the mesoderm just anterior to the mandible, then become part of the mid-gut wall, but later lose association with it. They survive even into the imago. The corpora allata do not arise from the ectoderm, but from the inferior wall of the antennary coelomic sac. The dorsal blood-vessel and associated tissues arise from the dorso-lateral walls of the two rows of coelomic sacs, which meet in the mid-line above the gut and enclose a tube. The cephalic aorta is derived from the antennary coelomic sacs. The blood-cells are formed exclusively from a narrow median ridge of mesodermal cells above the nerve-cord. The tracheal system arises from ten pairs of stigmatic invaginations from the prothoracic to the seventh abdominal segment. At their blind ends these expand to form the two main longitudinal vessels ; from their blind ends the branching tracheae to the tissues also grow out. In the advanced embryo all but the first and last stigmatic openings close. The nerve-cord arises early as a pair of ventral thickenings of the outer layer (lateral cords), within which the cells differentiate into dermatoblasts and neuroblasts. The latter are teloblasts, and bud off a succession of nerve-cells, which themselves further divide. The median cord, between the two lateral cords, contributes to the formation of the ganglia; its intersegmental neuroblasts attach themselves to the posteromedian wall of the ganglia, while the intra-segmental (neurogenic) cells form the roof of the completed ganglia, their axons contributing to the formation of the transverse commissures and longitudinal connectives. Sixteen ganglia form in the nerve-cord, the first three uniting into the sub-oesophageal ganglion, while the last three abdominal also fuse. The three component ganglia of the brain are clearly defined in the embryo. In the protocerebral ganglion the usual three lobes are seen. The optic ganglion does not contain neuroblasts and arises by invagination from the surface. Unlike the ventral nerve-cord, there are no median cord components in the brain. The stomatogastric system arises as three invaginations from the roof of the stomodaeum. The neurilemma of the ventral nerve-cord is derived from certain intersegmental median-cord cells. In the brain and sympathetic it is derived from the ganglia themselves. On the third day the germ-cells move forward in the abdomen, enter the hinder coelomic sacs and press forward as a solid cord of cells to the third abdominal segment. They form here a spherical mass of cells and are now encased in fat-body. The genital ducts arise from splanchnic mesoderm cells ensheathing the gonads. A peculiar cytological phenomenon—paracytoid formation—appears at the time of germ-band formation and in later phases of development. It is indistinguishable from the ‘chromatic globule extrusion’ already described from the metamorphosis of Calandra. Its significance is unknown.