GENERAL SKETCH 13 [ 



earlier observers, from the time of Martin Barry ('43) onwards, had 

 seen the spermatozoa inside the egg-envelopes, or asserted its entrance 

 into the egg. 



In many cases the entire spermatozoon enters the egg (mollusks, 

 insects, nematodes, some annelids, Petromyzon, axolotl, etc.), and in 

 such cases the long flagellum may sometimes be seen coiled within 

 the egg (Fig. 64). Only the nucleus and middle-piece, however, are 

 concerned in the actual fertilization ; and there are some cases 

 (echinoderms) in which the tail is left outside the egg. At or near 



P.b. 



Fig. 64. Fertilization of the egg of the snail Physa. [KOSTANECKI and WIERZEJSKI.] 

 A. The entire spermatozoon lies in the egg, its nucleus at the right, flagellum at the left, while 

 the minute sperm-amphiaster occupies the position of the middle-piece. The first polar body has 

 been formed, the second is forming. B. The enlarged sperm-nucleus and sperm-amphiaster lie 

 near the centre ; second polar body forming and the first dividing. The egg-centrosomes and 

 asters afterwards disappear, their place being taken by those of the spermatozoon. 



the time of fertilization, the egg successively segments off at the upper 

 pole two minute cells, known as the polar bodies (Figs. 64, 65, 89) or 

 directive corpuscles, which degenerate and take no part in the subse- 

 quent development. This phenomenon takes place, as a rule, imme- 

 diately after entrance of the spermatozoon. It may, however, occur 

 before the spermatozoon enters, and it forms no part of the process 

 of fertilization proper. It is merely the final act in the process of 

 maturation, by which the egg is prepared for fertilization, and we 

 may defer its consideration to the following chapter. 



