34 ARTHUR DENDT. 



Of course these embryos exhibit a certain amount of variation 

 amongst themselves, but not in my opinion sufficient to make 

 it necessary to separate them into distinct stages. I have 

 accordingly selected No. 44 as typical, and propose to describe 

 it in some detail, referring afterwards to such deviations from 

 the type as seem to deserve special mention in the others. 



General Relations of the Blastoderm and Embryo. 

 — The living embryo was still very transparent, but a red- 

 coloured sinus terminalis was conspicuous in the blastoderm 

 surrounding the hinder part of the body (fig. 58, S. T.). The 

 sinus terminalis forms the margin of the posterior half of a 

 clear-looking area in which the embryo lies. This clear area 

 exhibits a strong constriction in the middle, where the vitelline 

 veins are developing (fig. 56). In some embryos (e.g. No. 46, 

 fig. 54, and No. 60, fig. 53) the clear area thus acquires a very 

 characteristic 8-shaped outline at this stage ; in No. 44, how- 

 ever, the shape is less regular. 



The part of the blastoderm which forms the front half of the 

 8, corresponding to part of the original area pellucida, lies 

 entirely above the anterior half of the embryo, which projects 

 freely beneath it, and in front of the vitelline veins. This 

 part of the blastoderm is thin, and contains but little yolk, 

 hence its transparency. It is composed of epil))ast and lower- 

 layer cells. Some of the latter (mesoblastic) adhere to the epi- 

 blast to form with it the serous envelope, which, however, is as 

 yet only partially separated from the underlying yolk-sac com- 

 posed of the remainder of the lower-layer cells. At the margin 

 of this anterior half of the clear area the lower layer (yolk-sac) 

 thickens suddenly and becomes heavily laden with yolk, Avhich 

 accounts for its greater opacity. At the level of the vitelline 

 veins the serous envelope and yolk-sac join the amnion and 

 pro-amnion, thus giving rise to the transverse crease across 

 the back of the embryo (fig. 56, Tr.L. ; com[)are also fig. 50, 

 Stage G). 



In the hinder part of the 8-shaped clear area, bounded by 

 the sinus terminalis, the serous envelope is separated from the 

 vascular yolk-sac by a wide pleuro-peritoneal space on each 



