Development of Chick 



45 



or the primitive knot, but posteriorly it gradually shallows out 

 into a rather expanded thickening which forms the end of the 

 streak. 



In front of the primitive streak the wide thickened area of the 

 ectoderm is now more sharply marked out from the rest of the 

 ectoderm. It constitutes the neural plate and is traversed longi- 



Fig. 25 after Duval. Chick at llth hour of incubation. 



C, surface view; A and B, sections taken along the lines A B and C D 

 respectively, ect ectoderm ; mes mesoderm ; end endoderm ; sg subgerminal 

 cavity; y yolk; ps primitive streak; ap area pellucida; ao area opaca. 



tudinally by a rather wide shallow groove, the neural groove. 

 This groove is absent just in front so that posteriorly the neural 

 plate is partly divided by the shallow neural groove, but in front 

 the two halves are united. 



The study of a series of transverse sections makes the features 

 peculiar to all these several parts clear and also explains their 

 relations to the whole egg and to each other. 



