Manchester Memoirs, Vol. I. (1906), No. 3. 



49 



the notochord and mesoderm, as in Lacerta and some 

 other Reptiles, is perhaps an open question ; this was 



3 



Fig. 29. 



Two stages in the formation of the primitive streak and groove in the 



Chick ; transverse sections. 



(Original.) 



A. 12 hours ; proliferation of cells in the upper layer of the blastoderm. 



B. 15 hours ; formation of lateral lips to the blastopore (primitive 

 groove) ; a diminutive yolk-plug is seen between them ; mesoderm is 

 spreading from the sides of the groove between the upper layer and the 

 paraderm, and the whole is fusing secondarily with the latter. 



B 

 Fig. 30. 

 Sagittal section of the blastoderm of the Sparrow (after Schauinsland). 

 A. Anterior half. B. Posterior half. 



In front of the dorsal lip the notochord has been produced ; below it is 

 a slight archenteric cavity. 



There is a slight ventral lip with a posterior tongue of mesoderm 

 {mes.v.) behind it. The cells of the primitive streak {p.s.) have fused with 

 the paraderm [p.d.). 



Stated by Balfour to be the case in the chick, but denied 

 by Kolliker. 



