16 ART. ]]. — S. HATTA. 



a vertical section through the two poles of the blastula which 

 is still spherical in outline. The segmentation cavity [s.c.) which 

 is semilunar in shape occupies the upper half of the ovum ; the 

 cavity is roofed by a dome-like layer, while its floor is formed 

 by a thick mass of cells. The cupola part (a.h.) is the animal 

 hemisphere which looks translucent, when observed as an 

 opaque object, because it is thin, consisting of a layer only 

 2-3 cells in thickness ; these cells are smaller than those 

 which compose the remaining part of the ovum, the vegetative 

 hemisphere (v.h.). The smaller cells, the micromeres or animal 

 cells as they are called, can further be distinguished from 

 the larger cells by their smaller nuclei and by the smaller quantity 

 of yolk-granules which they contain. The macromeres, which 

 contain larger nuclei, are loaded with an enormous quantity of 

 yolk.^) 



The segmentation cavity is not bordered by a sharp limit ; 

 the cells in both the animal and vegetative heuiispheres project, 

 or even are set free, into the cavity : in this stage, the cells of 

 both kinds are evidently still undergoing segmentation. 



In the vertical section {M(/. 13) through the axial plane of 

 the ellipsoidal ovum (cf. Fig 2), the segmentation cavity (s.c.) 

 shows conspicuous changes ; it is no longer semilunar, but circu- 

 lar in outline ; and it now occupies the upper four-fifths of the ovum. 

 The micromeric wall {micm.) forming the animal hemisphere is 

 still represented by a layer which is 2-3 cells thick ; the 

 vegetative hemisphere is, on the otlier hand, formed as before of 



1) B. Lwofl" (Ueber die Bildung der primllren Keimblätter etc. bei den Wirbeltieren. 

 Bull, Soc. Natural. Moscou, 1S94) informs that tjie yolk-granules contained in the macromeres 

 are coarser than those in tiie micromeres, a distinction which I can not make out in spite 

 of careful observations. 



