EMBRYOLOGY. 241 



In the next stage segmentation has commenced, resulting in twenty-eight 

 cells (Plate 13, fig. 13). These show no differentiation into micromeres and 

 macromeres, and so far as may be judged from sections the inequality of size 

 noticed among the cells is irregular and not confined to a definite hemisphere. 

 All of the cells are closely crowded together and consequently no sign of a blasto- 

 cele exists (Plate 14, fig. 11). These facts together with the absence of polar 

 bodies in this particular embryo and the uniform distribution of yolk renders 

 it impossible to distinguish the principal axes. 



In the succeeding stage (Plate 13, fig. 6) approximately one hundred nuclei 

 are present and an elongation of the larva defines the principal axis. There 

 are, however, no definite signs of blastopore or blastocele, and the size and 

 arrangement of the cells does not certainly define the dorsal and ventral surfaces. 

 Sections (Plate 14, fig. 8) show that at this stage several cells are wholly enclosed 

 within the partially formed external layer whose component cells are of unequal 

 size and irregular arrangement. In a few cases the position of karyokinetic 

 spindles indicates that the internal cells are formed by tangential divisions, a 

 species of delamination, while other cleavages at right angles to these further 

 increase the number of cells on the exterior. 



In advance it may be said that in the larvae of this species test cells partially 

 enclose the body as Pruvot ('90) has shown to be the case in Myzomenia banyu- 

 le7isis. While no definite reliance can be placed on reconstructions for determin- 

 ing the exact shape and arrangement of cells it nevertheless appears fairly well 

 established that in the stage under consideration the external cells are not 

 so definitely arranged as in the later stages, and even there they are not so 

 diagrammatically placed as in Pruvot's figures. 



In the next stage (Plate 13, fig. 12) a single polar body remains in place, 

 held by the \ntelline membrane, and the animal pole is thus determined, together 

 with the probable point of origin of the cerebral ganglia though these last named 

 structures have not as yet put in an appearance. The differentiation of the 

 test cells has become evident to a certain extent, although their exact limits 

 have not been determined from reconstructions. Granted that the polar body 

 has not shifted from its point of origin, it does not appear to mark the centre of 

 the test cells which may be seen (Plate 14, fig. 1) to extend over nearly the entire 

 dorsal surface. Furthermore while the test cells may form rows or definite 

 bands about the embryo, reconstructions give no clear evidence of this fact up 

 to the present point in the development. 



In sections the cells, exclusive of those forming the test, that is those destined 



