132 RTOHARD ASSHETON. 



Figs. 20 to 34 are all camera drawings, and each is magnified 

 465 times. 



Figs. 20 to 25 show sections through the whole embryo, the 

 subsequent figures through the embryonic disc, or a part of it 

 only. 



Fig. 20, and also fig. 16, are sections of silver nitrate 

 preparations. 



In both cases there is no difference between the cells at 

 the surface and those towards the centre as regards their 

 colouring or nuclei. Those inside are certainly more com- 

 pressed than those on the surface. So also it frequently 

 happens that a large segment may be so placed that although 

 the greater part of it may be said to belong to the " inner 

 mass/' yet a small part of it may appear on the surface, as x. 

 in fig. 20. 



If such a specimen is examined in optical section, this 

 individual cell might very likely give rise to such an appear- 

 ance as van Beneden describes, and lead to the idea of an inner 

 mass partially surrounded by an outer layer. The difference 

 in colour which van Beneden describes is totally absent in real 

 sections, and I can find no greater opacity of the inner mass in 

 optical section than what can be equally well explained by the 

 greater thickness through which the light must of necessity 

 pass in viewing a sphere in optical sections. I am not able to 

 offer any explanation of the condition figured by van Beneden 

 in his second figure ; I can only say that I have have not been 

 able to find it. 



Fig. 21 is a section through an unusually large specimen. 

 This specimen was preserved in Perenyi. This specimen I 

 believe shows the earliest stage in the formation of the cavity 

 of the blastodermic vesicle (C BL.). 



There is as yet no evidence of an internal hydrostatic 

 pressure, but the outer cells form a compact layer, and seem 

 at one- point to be as it were lifted away from the inner cells, 

 leaving a slight cleft (C. BL.). 



In fig. 22 this cleft has increased considerably. It must be 

 noticed that it does not extend through more than about 240°. 



