THE MORPHOLOGY OF FERTILIZATION 51 



more may enter a considerable percentage of eggs; 

 and the same result may be obtained to a lesser extent 

 by very heavy insemination. Such polyspermy in 

 normally monospermic ova leads to abnormalities which 

 soon result in the death of the eggs. There are, however, 

 certain species which are normally polyspermic; most 

 of these, as, e.g., Selachia, reptiles, and birds have very 

 large ova. These questions will also be discussed later. 



The actual penetration of the spermatozoon into 

 the ovum was first observed in the sea urchin by Fol, 

 in 1876. (See discussion in chap, i, p. 16.) According 

 to his account the first spermatozoon which comes in 

 contact with the gelatinous layer that surrounds the 

 egg enters it at once and its point comes in contact 

 with the egg, usually within a second or two. The move- 

 ments of the tail then slacken and the head of the sperm 

 elongates and enters the egg. The tail remains visible 

 for some seconds; then it disappears from sight. The 

 head of the sperm forms a small nucleus, "male pro- 

 nucleus," within the egg. In the starfish he described 

 a small protuberance of clear protoplasm, the "fertili- 

 zation cone," arising from the surface of the egg at the 

 point of contact of the spermatozoon, lasting but a 

 few seconds, and appearing to aid in the engulfing of 

 the spermatozoon in the egg. 



In most forms the process of penetration is so rapid 

 that the details are not readily observed; in Nereis, 

 however, I have found a form in which the final pene- 

 tration of the spermatozoon does not occur until about 

 fifty minutes after insemination, in which, therefore, 

 all the details of the penetration may be observed 

 (Figs. 2 and 3). The egg is provided with a tough 



