GERM-HISTORY OF THE LAXCELET. 443 



Fig. 17. p. 173.) Fertilization is accomplished in this way : 

 the moving whip-cells of the sperm approach the egg, and 

 with their head-portion, that is, the thickened portion of 

 the cell which encloses the nucleus, they force their way 

 into the yelk-mass or cell-substance of the egg. 



Either before or immediately after fertilization, the egg- 

 cell loses its original kernel, and appears for a time in the 

 form of a kernel-less cytod, as a monerula. (Cf. Fig. 19, p. 179.) 

 A new kernel soon, however, originates in the impregnated 

 yelk; this is the parent-kernel, and the monerula thus changes 

 into the parent-cell (cytula, Fig. 21, p. 181.) This now 

 undergoes a regular and total cleavage, the details of which 

 in a coral (Monoxenia) we have described in detail (cf. Fig. 

 22}. The repeated bisection of the parent-cell into 2, 4, 8, 

 16, 32, 64 cells and so on, gives rise to the globular, black- 

 berry or mulberry-shaped body which we called the " mul- 

 berry-germ" (morula, Fig. 22, N). Fluid collects in the 

 interior of this globular mass, composed entirely of one sort 

 of cleavage-cells, and the result is the formation of a spheri- 

 cal vesicle, the wall of which is composed of a single layer 

 of cells (Plate X. Fig. 9). We called this vesicle the mem- 

 branous germ-vesicle (blastula). Its contents fonn a clear 

 fluid ; the wall, which consists of a single layer of cells, is 

 the germ-membrane, or blastoderma (Fig. 22, F, G~). 



These processes take place so rapidly in the Amphioxus, 

 that in from four to five hours after impregnation, that is, 

 about midnight, the spherical blastula is complete. On one 

 .side of the latter appears a groove-like depression, by which 

 the vesicle is turned into itself (Fig. 22, H, p. 190). This 

 furrow grows constantly deeper, while the spherical form of 

 the vesicle changes into an oval or ellipsoid shape (Fig. 155). 



