26 BIOLOGICAL LECTURES. 



principle is illustrated throughout the whole history of the 

 cleavage ; whenever the nucleus is moved from one position to 

 another in the cell it is preceded by its aster. The method by 

 which the asters draw the two pronuclei together is, I believe, 

 by the formation, attachment and contraction of the astral or 

 archoplasmic fibrils. In this connection it is interesting to 

 contrast the methods by which the male and female cells 

 approach each other before and after the spermatozoon has 

 reached the ovum. In the case of Crepidula, as in most 

 other forms, the spermatozoon moves toward the ovum by the 

 lashing from side to side of a long thread-like flagellum. As 

 soon, however, as the sperm enters the ovum this flagellum is 



FIG. 6. Ovum of C. plana from FIG. 7. Separation of the two 

 upper pole ;' first contact of the two asters; the polar bodies lie immediately 

 pronuclei. over the female pronucleus. 



lost in most cases, and the rest of the progress toward the egg 

 nucleus and aster must be accomplished in some other way. 

 This, as just said, seems to be done by the formation and con- 

 traction of astral fibers. This movement of the sperm cell 

 within the ovum is the result of protoplasmic contractility 

 no less than is the movement of the spermatozoon outside of 

 of the nucleus, operating, however, in a slightly different way. 

 After the two asters have come in contact, they move slightly 

 to one side, remaining however connected, though individually 

 distinct, and the two pronuclei then come together, Fig. 6. 

 Even at this time, the sperm aster and nucleus are usually a 



