254 OUTLINES OF CHORDATE DEVELOPMENT 



the mesoderm forms a loosely arranged sheet of cells, thinning 

 peripherally and extending for a short distance between the 

 ectoderm and germ wall. 



The conditions found near the extremities of the primitive 

 streak deserve special mention. Posteriorly, the primitive 

 plate shows a transversely extended region of mesoderm pro- 



mp 





en 



FIG. 94. Transverse sections through various levels of the blastoderm at 

 different ages. A. Through the head process of an embryo of about sixteen 

 hours. After Duval. B. Through the primitive streak of an embryo of seven- 

 teen and one-half hours. After Lillie. C. Through the primitive plate of an 

 embryo of about the same age as A. After Lillie. ec, Ectoderm; en, endoderm; 

 /, primitive folds; g, primitive groove; gw, germ wall; m, mesoderm; mp, medullary 

 plate; pp, primitive plate. 



liferation, which continues across the posterior side of the 

 primitive plate, so that the pellucid area behind the primitive 

 streak is composed of all three germ layers (Fig. 94, C). In the 

 region of the head process, i.e., in front of the primitive pit 

 and streak, conditions are not essentially unlike those found 

 farther back. The primitive groove is absent, and the cells 

 of the three layers are more intimately fused. Relatively 



