52 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



As regards the 

 Birds, which of the 

 higher Anmiotes alone 

 concern us in this 

 volume, there is com- 

 plete agreement that 

 they are to be looked 

 on as highly-special- 

 ized descendants of 

 Eeptilian ancestors. 

 It follows therefore 

 that their develop- 

 mental phenomena 

 should be considered 

 in relation to the cor- 

 responding pheno- 

 mena in Eeptiles. 



Leaving out of 

 account certain vague 

 phenomena which 

 have been inter- 

 preted, in the present 

 writer's opinion un- 

 justifiably, as remin- 

 iscences of gastrula- 

 tion processes (see 

 Chapter X.), the for- 

 mation of a gastrular 

 lip seems to have 



Fig. 32. — Transverse sec- 

 tions through region of 

 neurenterie canal of Ohe- 

 loiiia embryo with about 

 16 segments. (After 

 Mitsukuri, 1896.) 



The mid-dorsal ectoderm has 

 become covered in to form the 

 neural tube (s.c.) as will be de- 

 scribed in Chap. II. Fig. C 

 shows the neurenterie canal 

 opening upwards through this, 

 while Figs. B, D and E, taken 

 from sections anterior and pos- 

 terior to the neurenterie open- 

 ing, show the continuity of 

 tissue from ectoderm to endo- 

 derm which is a characteristic 

 feature of a primitive streak. 

 ect, ectoderm ; end, endoderm ; 

 mes, mesoderm ; N, notochord; 

 ne.c, neurenterie canal ; s.c, 

 spinal cord. 



