72 



PKYKLOI'MKNT OF TllK FKOti'S EGG 



[Tn. VI 



embryo, we liiul that the layer of mesoderm is inserted be- 

 tween eetoderm and yolk-eells over all the posterior half of 

 the embryo. There is a small anlero-veiitral region into 

 whieh the luesodenu does not extend. Al a point posterior to 

 the section deserihed above, we tind the mesoderm extending 

 nineh farther ccntmUji* so as to nearly eneiivle this region ot 

 the embryo. The blastopore is completely eneireled by the 

 sheet of mesoderm. 



V),; 26 \. 1 onuituilinal sertion through a vounu embryo of Kaua. H, C. t>. 

 K. I'ross-sivtions of last in planes of lines in A. 



Tross-seetions through an older embryo are drawn in Ki^. '_!(>. 

 T>. (\ 1>, K. The embryo has flattened alon^ the mid-dorsal line. 

 The eetoilerm has heeome thinner along* this line, where a taint 

 o-roove ran be seen on the snrfaee of the living egg. the primi- 

 tive groove. On eaeh side of the mid-dorsal line, the eetoderm 

 is somewhat thieker than before, and the eells are more elosely 

 paeked together. The eetoderm oxer the snrfaee of the embryo 

 eonsists of an outer layer and of several inner layers ot eells. 

 The eavitv of the arehenteron has opened out ami is very large. 



