52 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



As regards the 

 Birds, which of the 

 higher Aumiotes alone 

 concern us in this 

 volume, there is com- 

 plete agreement that 

 they are to be looked 

 on as highly-special- 

 ized descendants of 

 Reptilian ancestors. 

 It follows therefore 

 that their develop- 

 mental phenomena 

 should be considered 

 in relation to the cor- 

 responding pheno- 

 mena in Reptiles. 



Leaving out of 

 account certain vague 

 phenomena which 

 have been inter- 

 preted, in the present 

 writer's opinion un- 

 justifiably, as remin- 

 iscences of gastr ula- 

 tion - processes (see 

 Chapter X.), the for- 

 mation of a gastrular 

 lip seems to have 



FIG. 32. Transverse sec- 

 tions through region of 

 iifurenteric canal of ( 1 h<-- 

 I n nia embryo with about 

 16 segments. (Alter 

 Mitsukuri, 1896.) 



The mid -dorsal ect"deim has 

 become covered in to form the 

 neural tube (*.<.) as will be de- 

 scribed in Chap. II. Ki.u'. C 

 shows the lieu rent eric canal 

 opening upwards ihroiiiih this, 

 while Fi-s. II. 1) ami K. taken 

 from section-, anterior and pos- 

 lei jiir to the neiirentei ie open- 

 in-, show the continuity of 

 tissue from ectoderm to endo- 

 der in \\hieh is a cliaiactei i>t ie 

 feature of a primitive streak. 

 iddei 111 ; . n,l, I'lidodemi ; 



natodarn ; N , notochoni; 



ne.C, Mem-enteric caiia: 

 spinal coid. 



