88 THE BIOLOGY OF THE FROG chap. 



trates the egg substance. The entrance of one spermato- 

 zoon seems to cause some change in the substance of the 

 egg whereby other spermatozoa are prevented from entering 

 it, as normally an egg is fertiHzed by only one 

 sperm cell, although there may be thousands 

 of others in the immediate vicinity. When 

 the head of the spermatozoon has entered 

 the egg, it begins to enlarge, and its nucleus, 

 which is now known as the male pronucleus, 

 assumes a spherical form. It migrates slowly 

 toward the central part of the egg, dragging 

 in behind it a mass of pigment granules from 

 the periphery, so that its course comes to be 

 marked by a dark streak. Before the male 

 pronucleus has penetrated very far the pro- 

 cess of maturation is brought to completion 

 by the formation of the second polar body. 

 The nuclear material remaining in the ^%% 

 after this second maturation division goes 

 into a resting stage, forming the female pro- 

 nucleus, or the nucleus of the matured ovum. 

 Fig. 15. — Sper- The male and female pronuclei approach 



matozoon of ^^^ ^^^jj ^^^^^ -^^^^ ^^j^^^^ j^ ^^^^^ ^j^^ 



Nana esculen- •' 



ta. (After La copulatioH iiucleus. The number of chromo- 

 Vaiette St. ggmes contributed by both parents to the 

 nucleus of the fertilized egg is the same, and 

 this has doubtless a fundamental relation to the fact that, 

 on the average, offspring inherit qualities from both their 

 parent forms in an equal degree. This correlation of 

 equality of inheritance from the two parents with the 

 equality of their contributions of chromatin material to the 

 fertilized eggs lends strong support to the view that it is to 

 the chromatin of the nucleus that we must look for the 



