EARLY DEVELOPMENT OF THE LA.CERTILIA. 655 



extreme end of the embryo, must necessarily, with reference to 

 the embryo, be the hindermost section of the blastopore, and 

 therefore the part of the blastopore apparently behind this can 

 only be so owing to the embryo not being folded off from the 

 yolk sac ; and as the yolk sac is in reality a specialised part of 

 the ventral wall of the body, the yolk blastopore must also be 

 situated on the ventral side of the embryo. 



Kolliker and other distinguished embryologists have believed 

 that the epiblast of the whole of the primitive streak became 

 part of the neural plate. If this view were correct, which is 

 accepted even by Rauber, the hypothesis I am attempting to 

 establish would fall to the ground. I have, however, no doubt 

 that these embryologists are mistaken. The very careful ob- 

 servations of Gasser shew that the part of the primitive streak 

 adjoining the embryo becomes converted into the tail-swelling, 

 and that the posterior part is folded in on the ventral side of the 

 embryo, and, losing its characteristic structure, forms part of the 

 ventral wall of the body. On this point my own observations 

 confirm those of Gasser. In the lizard the early appearance of 

 the neurenteric canal at the front end of the primitive streak 

 clearly shews that here also the primitive streak can take no 

 share in forming the neural plate. 



The above considerations appear to me sufficient to establish 

 my hypothesis with reference to the nature of the primitive 

 streak, which has the merit of explaining, not only the structural 

 peculiarities of the primitive streak, but also the otherwise inex- 

 plicable position of the embryo of the amniotic vertebrates in 

 the centre of the blastoderm. 



