50 J. P. HILL. 



the conclusion reached above, viz. that the formative hemi- 

 sphere is derived from the smaller-celled i-ing of the 16-celled 

 stage. On that conclusion is based my interpretation of the 

 poles in the unsegmented ovum, and of the two cell-rings of 

 the 16-celled stage as respectively npper and lower. 



Of vesicles over 1 mm. in diameter, the smallest in which I 

 have been able to detect the sutural line above referred to 

 measure 3"25 mm. in diameter. In three lots of vesicles, 3*5 

 mm. in diameter from three different females, I have failed to 

 recognise it, whilst in two other lots, respectiveh' o'75 mm. 

 (average) and 4 mm. in diameter, the line appears to be in 

 course of differentiation as in the o"25 mm. vesicles. A 

 portion of the wall of one of the 3"5 mm. vesicles just referred 

 to is shown in PI. 4, fig. 41, and a portion of the wall of the 

 3*25 mm. stage, including the sutural line, in fig. 42. Both 

 vesicles w^ere fixed in the same fluid, viz. picro-nitro-osmic 

 acid. Comparison of the two figures reveals the existence, quite 

 apart from the presence of the junctional line in fig. 42, and its 

 absence in fig. 41, of certain more or less obvious diffei"ences 

 between them. In fig. 41 the cells are larger, and their cyto- 

 plasmic bodies ;i.re inconspicuous, being fairly homogeneous 

 and lightly staining. In fig. 42, on the contrary, the cell- 

 bodies are strongly marked, the cytoplasm being distinguish- 

 able into a lighter-staining peripheral zone, and a much more 

 deeply staining perinuclear zone, showing evidence of intense 

 metat)olic activity. This latter zone is more or less vacuolated, 

 and contains, besides larger lightly staining granules, numerous 

 smaller ones of varying size, stained brown by the osmic acid 

 of the fixative. In the 4 mm. vesicles the cells show precisely 

 the same characters; in the 3'75 mm. vesicles, Avhich were 

 fixed in a picro-corrosive-acetic fluid, the granules are absent 

 from the cytoplasm, otherwise the cells are similar to those 

 of the other two. Mitotic figures are common. The sutural 

 line is recognisable in all three sets of vesicles (3*25, 3"75, and 

 4 mm.) (fig. 42, j.L), but I cannot be certain that it runs con- 

 tinuously round, and it appears to have a rather more sinuous 

 course than in later blastocysts. The cells of the two regions 



