STUDIES ON SEX IN CREPIDULA 21 



which is distinct from the spermatogonium and which is be- 

 Heved to have a different origin, the spermatoblast in Crepidula 

 arises from a cell which cannot be distinguished from the sperma- 

 togonia of the true sperm; the cell will first be called a sperma- 

 toblast when it can be distinguished from those which are de- 

 stined to undergo the maturation divisions. 



Development of the apyrene sperm in Crepidula plana. After 

 the spermatogonia which have been cut off from the peripheral 

 layer of the gonad have undergone a number of divisions in the 

 lumen, the progeny differentiate into primary spermatocytes of 

 the eupyrene sperm, and spermatoblasts of the apyrene sperm. 

 Figure 4 represents a section through an immature male gonad 

 in which the a.pyrene spermatoblasts {sph) are just beginning to 

 be distinguishable. They are about the same size as the cells 

 which are to grow into the primary spermatocytes (spcl), but 

 their relation of cytoplasm to nucleus is different. In the sper- 

 matogonia and spermatocytes of the true sperm, the nucleus 

 takes up most of the body of the cell, the cytoplasm being re- 

 stricted to a thin layer. In the spermatoblast the nucleus is 

 smaller and lies eccentrically. The spermatoblast contains a cy- 

 toplasmic granule which is more prominent in the atypical cells 

 than in the typical, though it occurs in all types of germ cells. 

 This body, the nature of which is not clear, disappears during 

 the later changes in the cell and leaves no trace. It is more fully 

 discussed in the description of figure 4. 



In the section through a young testis shown in figure 6 the 

 differentiation of the typical and atypical series is more marked 

 than in figure 4. Both the young eupyrene spermatocyte {spcl, 

 at top of figure) and the apyrene spermatoblast (sph) experience 

 a certain amount of growth at this time; but the growth of the 

 spermatocyte is chiefly a nuclear growth, while that of the sper- 

 matoblast consists in an increase of cytoplasm, the nucleus re- 

 maining small. The chromatin of the spermatocyte begins to 

 form the leptotene thread. The chromatin of the spermato- 

 blast becomes aggregated in small masses, and the reticular 

 condition of the nucleus, indistinct in the spermatogonial stage, 

 can no longer be made out. 



