THE LAEVA OF ASTERIAS VULGARIS. 107 



epithelium is very much thicker, and crowded with nuclei of 

 various sizes, which will form the next crop of eggs. 



Spermatogenesis. — The testis (fig. 3) differs histologically 

 from the ovary only in a less development of the muscular 

 layer {m.) of its wall, and in the smaller size of the cells of the 

 germinal epithelium {g. e.). These cells separate from the 

 epithelium, and by the increase in size and final separation of 

 the germinal epithelium-cells behind them are pushed towards 

 the centre of the lumen. These separated cells are the sperm 

 mother-cells of the spermatozoa (fig. 3, s. m. c). Their dia- 

 meter is many times less than that of the ova at a correspond- 

 ing stage. Each sperm mother-cell, in its progress towards 

 the centre of the lumen, divides into two smaller cells. In 

 the further progress towards the centre of the lumen each of 

 these smaller cells divides again into two. Thus from the 

 sperm mother-cells are formed four very small cells (fig. 3, sp.), 

 each of which, without further division, is directly changed 

 into the form characteristic of the spermatozoon. Each sperm 

 mother-cell gives rise to four spermatozoa, and not to a large 

 or indefinite number. The entire sperm mother-cell appa- 

 rently passes into the four spermatozoa, with no traces of the 

 " residual corpuscles " supposed to be the homologues of the 

 polar bodies. This fact is of interest when compared with 

 Hertwig's recent results upon Ascaris (15). 



Cleavage. — The facts of the process of maturation, fertili- 

 sation, and cleavage have been carefully studied by others. 

 Sufficient here to add that, as in many other groups, the four 

 cells arising by the first two divisions become pressed to- 

 gether, so that two have their apices directed towards the 

 centre, truncated, so to speak, while the other two have sharp 

 points; and the arrangement is such that the two opposite 

 cells are alike, while the two adjacent cells are unlike. 



The plane of bilateral symmetry is plainly indicated in the 

 8-cell stage (fig. 4). In the 16-cell stage the difference in 

 size between the cells of the ectodermal and entodermal area 

 is conspicuous (fig. 5). Throughout the process of cleavage I 

 watched carefully for some particular cell to which the origin 



