380 ARTHUR T. MASTEWMAN. 



stage shows (fig. 21) that there is a well-defiued blastocoslic 

 cavity, slightly compressed from side to side. Ronle (15) 

 states that the blastoccele does not develop till much later. 



The third furrow is at right angles to the first two, and is 

 almost exactly equatorial. The fact that the first four 

 blastomeres taper results in the upper four being less in bulk 

 than the lower (fig. 5). The base and apex of this stage 

 appear similar to those of the four-celled. The further stages 

 in segmentation are difficult to trace in the specimens to hand. 

 The cells of the blastula stage are very nearly if not quite 

 equal in size. The cells of the lower hemisphere, however, 

 appear to retain a slightly greater size. Fig. 22 is a median 

 section through an early blastula, with five upper cells and 

 four lower cells in the median plane. The blastula is still 

 pei'fectly spherical, but in further segmentation both the 

 blastula and the blastoccele appear elongated in a median 

 section (fig. 28). In this figure the lower cells are distinctly 

 larger and rather longer than the upper ones. The elongated 

 appearance is not due to an elongation in one axis of the 

 blastula (the horizontal axis), but to a gradual flattening of 

 the whole blastula in the vertical plane, so that it becomes 

 disc-shaped (cf. fig. 11). Fig. 24 shows an even later stage 

 of the blastula, in which thei'e is a great increase of cells, and 

 the lower cells have become slightly flattened on their outer 

 surface. This is the commencement of gastrulation. As 

 segmentation proceeds, the nuclei, which in the four-celled 

 stage lie at the centre of the cells, gradually move outwards 

 to the peripheral part, and in fig. 24 come to lie almost under 

 the limiting cell wall. 



Gastrulation and the Fate of the Blastopore. 



The external appeai'ance of the several stages here dealt 

 with is shown in figs. 6 to 15. The embryos not having 

 been observed alive, one can only follow the changes by the 

 examination of a selected series. The whole process of 

 gastrulation and closure of the blastopore appears to be 



