GASTKULA THEORY 



43 



the iuvagiiiation of one pole of an originally simple hollow sphere the blastula the invaginated 

 portion becoming the primitive entoderm and the remaining part of the wall of the vesicle 

 forming the primitive ectoderm (fig. 69). This condition, which was discovered by Kowalewsky, 

 is known as the gastrula stage, and it is regarded by many embryologists, following Haeckel, 

 as typical of the mode of formation of the bilaminar blastoderm throughout the animal 

 kingdom. The aperture by which the cavity of the gastrula, whether formed by delamination 

 or invagination, communicates for a time with the exterior has been termed the blastopore 

 (Lankester). 



There are not wanting observations in mammals pointing to the existence in the bilaminar 

 blastoderm of a blastoporic aperture. Thus in the mole Heape figures an aperture just before 

 the primitive streak appears (fig. 70). Similar figures have been published, though for rather 

 earlier stages, by Selenka in the opossum, by Hubrecht in the shrew and hedgehog, by Keibel in 

 the rabbit, and by Bonnet in the dog ; while in Tarsius the narrow slit in fig. 57, p. 39, may 

 have the same significance (Hubrecht). In other mammals, however, no such aperture has 



vilii of chorion 



notochordal 

 plate 



heart 



mesoderm - 



^SL chor-ion 



-'; mesoderm 



"~ embryophore 

 "~ primitive streak 



allaiitois 



yolk-sac 



entoderm 



*~ vessels 



Fiu. 65. MEDIAN LONGITUDINAL, SECTION OF THE HUMAN OVUM BEPBESENTED IN FIG. 72, p. 49. 

 (After Graf v. Spee, from Kollmann.) 



The wide and vertical neurenteric canal is seen opening from the amniotic cavity into the yolk-sac. 



been seen at this stage, and the opening in all cases has only a transitory existence. It is 

 doubtful how far it corresponds to the blastopore of a gastrula derived from a holoblastic ovum, 

 the whole of which is utilised for the formation of the embryo. 



The accumulation of yolk in the egg profoundly modifies the process of gastrulation. Thus in 

 the amphibian holoblastic egg, owing to the character of the segmentation, a considerable proper - 

 t ion of the yolk-laden entomeres are already within the blastula, and the invagination phenomena 

 by which the so-called gastrula is completed, represent a second phase of gastrulation concerned 

 in the formation of a dorsal plate which forms the dorsal wall of the trunk with the notochord 

 and segmented mesoderm. During the development of the plate the gastrula cavity (archenteron) 

 is formed by a partial invagination, and the breaking through of the space so produced into the 

 cavity of the blastula. The archenteron is slit-like, because the gastrula is almost entirely filled 

 by the entomeres (segmented yolk). In the lower Amniota the animal pole of the telolecithal egg 

 alone segments. The germinal disc in its early phases represents a small portion of the blastula- 

 wall, which is not theoretically completed tilt long afterwards, when the blastoderm has grown 



