THE FERTILIZATION OF THE OVUM. 



25 



center unite. The facts observed in the case of Rhinchehnis 

 speak against Fol's theory." 



In the face of the conclusions of these well-known investi- 

 gators, it is interesting to find, in the case of the Crepidula 

 egg, a well marked centrosome and surrounding parts of the 

 aster present at each pole of the spindle in the formation of 

 each of the polar bodies ; and immediately after the second 

 polar body has been extruded, and while the sperm nucleus 

 and aster are still far removed from the egg nucleus, a large 

 and distinct aster can be seen in contact with this nucleus. 

 This egg aster lies below the egg nucleus, and usually slightly 

 to one side of the chief axis of the ovum, as shown in Fig. 4. 



Fig. 4. — Ovum of C. plana, side view; FiG. 5. — Ovum of C. plana seen from 

 the asters are sliown in dotted outline, upper pole ; in tliis and the following 

 the clear spheres with dark centers, at figures the male aster and nucleus lie to 

 the upper pole, are the two polar bodies, the left, the female to the right. First 



contact of the two asters. 



It is at first much larger than the sperm aster, and in fact 

 remains larger until it enters upon "/cr MarcJic dc la Quadrille T 

 The sperm nucleus always approaches the ^g^ nucleus from 

 below and in such a way that the sperm aster is directed 

 toward the egg aster. Fig. 4. The force which draws the two 

 pronuclei together seems to exist between the two asters rather 

 than between the pronuclei. Accordingly, in the progress of 

 the sperm nucleus toward the Q^g nucleus, the asters first 

 come in contact. Fig. 5. In all movements of the nucleus, the 

 latter appears to be passive while the asters are active. This 



