So F. M. BALFOUR. 



The growth outwards of the mesoblast from the axis of the 

 primitive streak is probably a remnant of the invagination of the 

 hypoblast and mesoblast from the lip of the blastopore in 

 Amphibia, &c. 



The groove in the primitive streak may with great plausibility 

 be regarded as the indication of a depression which would natu- 

 rally be found along the line where the thickened edges of the 

 blastoderm became united. 



"With reference to the third condition, I will make the following 

 observations. The neurenteric canal, as it is placed at the 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 em- 

 bryologists are mistaken. The very careful observations of 

 Gasser show 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 shows 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. 



