418 WALTER HEAPE. 



fig. 2 was drawn being smaller than the fully-segmented ovum 

 and not larger, as would have been expected. 



I can only attribute this condition either to the variation in 

 size of different ova of the same age, of which fact I have abun- 

 dant evidence, or to the effect of the preserving fluid, although 

 in both instances the objects were treated with silver nitrate 

 and preserved in glycerine. 



However that may be — and this is the point which I wish 

 to emphasise — the size of the inner mass in fig. 2 is relatively 

 smaller than that in fig. 1, the diameter of the ovum (fig. 2) 

 being '12 mm., and that of the inner mass '06 mm. 



The ovum rapidly enlarges, and in fig. 3 the relation in size 

 of the whole vesicle to the remnant of the inner mass is repre- 

 sented in an early stage of the development of the blastodermic 

 vesicle. 



The vesicle in this specimen is *31 mm. and the inner mass 

 •04 mm. in diameter. 



This increase in size is due to some extent, without doubt, 

 to the flattening out and multiplication of outer layer cells 

 (vide figs. 16 — 19); but I believe that up to this point in 

 stage B the cells of the inner mass also contribute to that end. 



I have been unable clearly to substantiate this opinion by 

 means of sections, but the size of the inner mass in this spe- 

 cimen bears out my views; it is '02 mm. less in diameter than 

 the inner mass in the specimen figured in fig. 2, and '07 mm. 

 less than the inner mass of the fully-segmented ovum. Further, 

 I have made measurements of a considerable number of speci- 

 mens of a similar age, and have found this ratio to be almost 

 uniformly constant. 



The structure of the wall of the vesicle and of the inner mass 

 at this stage is seen in figs. 16 — 19. 



The vesicle wall is formed of much flattened polygonal cells 

 closely attached to the zona radiata, which bounds them on 

 their outer side. 



The cells contain a large nucleus situated in the centre, and 

 causing it to bulge towards the cavity of the vesicle. 



The nucleus in section appears to be of oval form, while in 



