194 RICHAED ASSTTETON. 



two parts which are very much less opaque, but still more opaque 

 than the extra-embryonic part of the blastodermic vesicle. 



The specimen of which fig. 4 is a drawing was cut into a series 

 of transverse sections, that of which fig. 4a is a drawing, into 

 a series of sagittal sections. Fig. 17, PI. 20, is a very nearly 

 median sagittal section of the specimen represented by fig. 4 a. 

 In this drawing A. is the anterior end, and P. the posterior. 



The epiblast {EP.) of the embryonic disc is thickened 

 throughout. The under or inner surface of the epiblast is even 

 along the anterior half, while along the posterior it presents a 

 very irregular, and towards the extreme posterior end, broken 

 edge. 



The hypoblast (HY.) is separate all along, and forms a con- 

 tinuous layer. It is specially thickened at the two points where 

 it underlies the extreme anterior and posterior ends respectively. 

 There are now between the hypoblast and the epiblast, towards 

 the posterior end, certain scattered cells, some apparently en- 

 tirely, others only partially separated from the epiblast. These 

 are the first mesoblast cells which make their appearance. 



The figs. 13 — 16 on PI. 21 are from transverse sections of 

 the specimen shown in fig. 4. They are drawn on a larger 

 scale, and, taken together with section 17, demonstrate pretty 

 accurately the structure of the specimens figs. 4 and 4 a. 



Fig. 13 is taken through the anterior part of the specimen 

 along the line 13 in fig. 4. The central portion only is given 

 in fig. 13 (see fig. 13, A.). This section carries out the evidence 

 derived from the longitudinal section (fig. 17), and shows that 

 the under or inner surface of the epiblast is quite smooth ante- 

 riorly. 



There is no median thickness of the hypoblast, and no sign 

 of mesoblast cells. In fact the anterior region of the embryonal 

 area has not greatly altered from the condition of the circular 

 embryonic disc upon the day previous. The posterior end, 

 however, is very different. Fig. 16 passes through the speci- 

 men (fig. 4) along the line 16 — that is to say, through the very 

 densest portion of the area. 



Here we see the epiblast enormously thickened, and a certain 



