Observations upon the Development of the Segmentation Cavity, the Archenteron, the Germinal Layers, and the Amnion in Mammals
Authors: Robinson Arthur 
Published: April 2021

ABSTRACT
A—General description of the development of the ova of the rat and mouse up to the period of completion of the blastodermic vesicle, and comparison with the results obtained by Professors Fraser, Duval, and Selenka.
Results.—There is a segmentation cavity, which is not the blastodermic cavity. The segmentation cavity disappears simultaneously with the appearance of the archenteron. The archenteron appears amidst the hypoblast. The young ovum consists principally of hypoblast, which becomes vacuolated to form the cavity of the yolk-sac. The cavity of the yolk-sac is never bounded by epiblast alone. The epiblast extends over the outer surface of the hypoblast, not the hypoblast over the inner surface of the epiblast. The hypoblast never becomes entirely surrounded by epiblast.
B—Description of the formation of the mesoblast.
Results.—The mesoblast is formed partly from the peristomal cells in the region of the primitive streak, partly from the embryonic hypoblast, and partly from the extra-embryonic hypoblast. Mesoblastic formation commences at the posterior end of the embryonic area, not anteriorly, as in the hedgehog. In the embryonic area the pericardial mesoblast is the last formed.
C—Description of the formation of the chorda dorsalis.
Results.—The chorda is formed entirely from the primitive hypoblast, to which it last remains adherent at the dorsal end of the bucco-pharyngeal membrane and the anterior end of the primitive streak. There is no “Kopffortsatz “of the primitive streak.
D—Comparison of the ova of the rat and mouse with the ova of other mammals and the lower vertebrates.
Conclusions.—The ova of mammals do not differ essentially from the ova of other vertebrates. They do not consist in the early stages of an epiblastic vesiole containing an inner mass of epiblast and hypoblast, bnt of a large hypoblastic mass which supports a small epiblastic disc. The ova of mammals present all the characteristic features of comparatively large-yolked ova.
E—Description of the formation of the amnion and discussion of the relation of amnion formation to “inversion.”
Conclusions.—There is a pro-amnion in the rat and mouse. Pro-amnion formation and “inversion” are distinct processes, and “inversion” is not precocious pro-amnion formation. The whole of the amnion is formed from the tail-fold.
Comparison of amnion formation in the rat and mouse with amnion formation in man and other mammals.
F—Description of the formation of the cœlom.
Results.—The cœlom commences bilaterally and in the embryonic area. The pericardial cœlom is an extension of the embryonic cœlom from behind forwards. It does not communicate with the anterior portion of the extra-embryonic cœlom.
The formation and the extension of the germinal layers, their relation to the segmentation cavity and archenteron, and the derivation of the chorda dorsalis in the mammalia are developmental problems which have initiated many investigations. From the results which have hitherto been obtained only very general conclusions can be drawn, and upon many important points the recorded observations are contradictory. This is all the more noteworthy inasmuch as many of the observations have been made upon the same kind of animal; others, however, upon animals belonging to widely divergent species.
The blastodermic cavity of the mammalian ovum lies, at first, between the epiblast and hypoblast and corresponds in position with the segmentation cavity of the lower Vertebrata. In the rabbit (2), rat (9, 13, 45), mouse (9, 13, 44), guinea-pig (45), shrew (23), mole (16), bat (4), and opossum (46) it is said to become surrounded by the hypoblast. If this actually occurs, then the mammalia are separated from all the other Vertebrata by a peculiarity which as yet has received no satisfactory explanation, for in none of the other Vertebrata does the segmentation cavity become surrounded by hypoblast.