124 



FUNDAMENTALS OF CYTOLOGY 



even while it is still separated from the membrane bj^ a layer of jelly 

 surrounding the egg; nevertheless the sperm enters, as though it were 

 being acted upon by some force resident in the egg. 



In a marine annelid worm (Nereis) it has been shown how the egg 

 behaves during the entrance of the sperm. If the sperm reaches the egg in 

 the brief period during which the egg is fertilizable, the union of the two 

 proceeds as follows (Fig. 92). All but one of the many sperms which 

 may have attached themselves to the egg are usually carried away from 

 its surface by a jelly which flows out from an alveolar zone just beneath 



Fig. 92.^-Entrance of spermatozoon into egg of N'ereis. Only the sperm head is drawn 

 in by the fertilization cone; the middle piece and tail remain outside. (From F. R. Lillie: 

 Problems of Fertilization, University of Chicago Press.) 



the membrane. From the inner region of the egg a transparent /pr^///2a- 

 tion cone then extends across this zone and touches the membrane at the 

 point where the single remaining sperm is about to penetrate it. The 

 perforatorium pierces the membrane and becomes attached to the cone 

 which is then withdrawn toward the inner region of the egg, carrying the 

 head of the sperm with. it. The sperm's middle piece and tail are left 

 outside the egg. In some sea urchins both head and middle piece enter, 

 while in most animals the whole sperm passes in. 



In different animals there is considerable variation in the stage of the 

 egg's maturation at the time when the sperm enters. In sea urchins and 

 starj&sh both meiotic divisions have been completed, the sperm thus 

 entering a fully matured egg. The frog's egg is entered during the meta- 

 phase of the second meiotic division. Entrance occurs during the first 

 metaphase in certain annelids, insects, and mollusks, and even slightly 



