204 



EMBRY0L0GICAL TYPES 



(peristomial) mesoderm. At the extreme front end of the 

 embryo, anterior to the region of the midbrain which is the 

 farthest spot reached by the proliferation of the primitive 

 pit and streak, a certain amount of mesoderm and the front 

 end of the notochord are formed by splitting off from the lower 

 layer. As in Amphioxus and the frog, therefore, the most 

 anterior part of the mesoderm (gastral) and of the notochord 

 are formed from the endoderm by delamination or splitting, 



Fig. 92. — Gallus : transverse sections through the blastoderm of a hen's egg 

 after 24 hours' incubation. (From Jenkinson.) 



A, through the posterior ; B, through the anterior region of the embryo. 

 en, endoderm ; m, mesoderm ; mg, neural groove ; mlp, unsegmented meso- 

 derm of the lateral plate ; mvp, segmented mesoderm (somites) of the 

 vertebral plate ; n, notochord. The posterior region of the embryo is 

 at a less advanced stage of development than the anterior region. 



while farther back they are formed as a result of the activity 

 of the cells of the lips of the blastopore (primitive streak). 



The upper layer of the blastoderm may now be called 

 ectoderm, the lower layer endoderm, and the mesoderm 

 extends out to the side between them. Gastrulation in the 

 chick therefore does not involve invagination. The endoderm 

 is formed precociously, probably so as to assist in digesting 

 the enormous quantity of yolk. 



Head-fold. — A very important thing to notice is that the 



