IMPREGNATION OF THE OVUM. 



81 



and it makes its way into the egg by passing through the promi- 

 nence. The tail of the spermatozoon, no longer motile, remains 

 visible for some time after the head has bored its way in, but its 

 place is soon taken by a pale conical body, which is, however, 

 probably in part a product of the metamorphosis of the tail 

 itself (fig. 32). It eventually becomes absorbed into the body of 

 the ovum. 



At the moment of contact between the spermatozoon and 

 the egg the outermost layer of the protoplasm of the latter 

 raises itself as a distinct membrane, which separates from the 

 egg and prevents the entrance of other spermatozoa. At the 

 point where the spermatozoon entered a crater-like opening is 

 left in the membrane, through which the metamorphosed tail of 

 the spermatozoon may at first be seen projecting (fig. 32). 



The head of the spermatozoon when in the egg forms a 

 nucleus, for which the name male 

 pronucleus may be conveniently 

 adopted. It grows in size, pro- 

 bably by assimilating material 

 from the ovum, and around it is 

 formed a clear space free from 

 yolk-spherules. Shortly after its 

 formation the protoplasm in its 

 neighbourhood assumes a radiate 

 arrangement (fig. 33). At what- 

 ever point of the egg the sperma- 

 tozoon may have entered, it grad- 

 ually travels towards the female 

 pronucleus. The latter, around 

 which the protoplasm no longer 

 has a radiate arrangement, re- 

 mains motionless till the rays of 



the male pronucleus come in contact with it, after which its 

 condition of repose is exchanged for one of activity, and it 

 rapidly approaches the male pronucleus, apparently by means 

 of its inherent amoeboid contractions, and eventually fuses with 

 it (figs. 3436). 



As the male pronucleus approaches the female the latter, 

 according to Selenka, sends out protoplasmic processes which 

 B. ii. 6 



FIG. 33. OVUM OF ASTERIAS 

 GLACIALIS, WITH MALE AND FEMALE 

 PRONUCLEUS AND A RADIAL STRIA- 

 TION OF THE PROTOPLASM AROUND 

 THE FORMER. (Copied from Fol.) 



