FERTILIZATION 181 



We left the sperm head and middle piece lying a short distance 

 below the surface of the egg. We may disregard the tail piece 

 now, for even in those cases in which it enters the egg it is left 

 behind the head and takes no active part in subsequent proc- 

 esses. The head and middle piece now move more slowly, 

 along a path which is, for a short distance at any rate, a 

 radius of the ovum. Then they separate slightly, and the two 

 rotate through approximately 180, so that the middle piece is 

 placed in advance of the head (Figs. 94, 95). There ensues a 

 considerable metamorphosis of these elements. The sperm 

 head loses its sharp outline and gradually enlarges; its outline 

 soon becomes very irregular and indistinct, and vacuoles 

 appear. Soon it has expanded into an organ of considerable 

 size and has acquired a typical nuclear structure with linin 

 network, chromatin granules, and nuclear membrane. In the 

 meantime the middle piece has undergone an even more exten- 

 sive transformation (Fig. 94). Before the sperm halts in its 

 inward progress, even before the rotation in some cases, the 

 middle piece has begun to dissolve and in connection with it 

 appears a centrosome, surrounding which a small aster appears. 

 During the pause of the sperm nucleus, the centrosome and 

 aster each divide into two, and the daughters diverge slightly 

 while the asters grow somewhat larger. 



It will be remembered that during the metamorphosis of the 

 spermatid into the spermatozoon, one or both of the centrosomes 

 of the former either passed into the middle piece, or actually 

 formed the larger part of it, although frequently no centrosome 

 is actually visible in the middle piece of the fully formed 

 spermatozoon. WTien the centrosome appears in the egg, in 

 connection with the middle piece of the entering spermatozoon, 

 it is possible, though not likely, that this is really the same 

 centrosome that was present in the spermatid, and that there 

 is consequently a genetic continuity of centrosomes, from gen- 

 eration to generation, as well as of nuclear components. Such 

 a continuity has, however, not been definitely observed. On 

 the other hand, it may be that the middle piece forms, either 

 from its own substance, or from that of the egg, a new centro- 



