254 PEOFESSOE F. M. BALFOUR. 



anterior opening of the blastopore, the embryonic mouth, has 

 increased remarkably in size. It is circular, and is placed 

 between the second pair of mesoblastic somites. The anterior 

 dilatation of the posterior opening of the blastopore, the em- 

 bryonic anus, has, like the anterior opening, become much 

 enlarged. It is circular, and is placed on the concavity of the 

 ventral flexure. From its hind end there is continued to the 

 hind end of the body a groove (shown in fig. 39 as a dotted 

 line), which we take to be the remains of the posterior slit-like 

 part of the posterior opening of the blastopore of the preceding 

 stage. The posterior dilatation has disappeared. The embryo 

 has apparently about thirteen somites, which are still quite 

 distinct from one another, and apparently do not communicate 

 at this stage with the mesenteron. 



The epiblast lying immediately over the somites is, as in the 

 earlier stages, thickened, and the thickenings of the two sides 

 join each other in front of the embryonic mouth, where the 

 anterior pair of mesoblastic somites (the prseoral lobes) are 

 almost in contact. 



The median ventral epiblast, i.e. the epiblast in the area, 

 bounded by the embryonic mouth and anus before and behind 

 and by the developing nerve-cords laterally, is extremely thin, 

 and consists of one layer of very flat cells. Over the dorsal 

 surface of the body the epiblast cells are cubical, and arranged 

 in one layer. 



Measurements of Embryo of Stage E. 



Lengtli of embryo ...... 1"12 mm. 



Greatest width . . . . • . 'Qi „ 



Distance from front end of embryonic mouth to hind end of 



embryonic anus ...... '48 „ 



Greatest length of embryonic mouth . . . . '16 „ 



Lengtli between hind end of embryonic mouth and front end 



of embryonic anus . . . . . *29 „ 



These measurements were made with a micrometer eyepiece, 

 with the embryo lying on its back in the position of fig. 38, so 



