xiv THE CONTINUANCE OF LIFE 311 



aperture in the shell. It works its way inwards until 

 its nuclear part unites with that of the ovum. The union 

 is again intimate and complete. 



The result of fertilisation is the intimate and orderly 

 union of the nucleus of the egg-cell and the nucleus of 

 the sperm-cell, but it must be remembered that there is 

 also a mingling of the infinitesimally minute cell-sub- 

 stance or cytoplasm of the spermatozoon with the rela- 

 tively large cell-substance of the ovum. Some idea both 

 of the orderly complexity of the nuclear union and of 

 the carefulness of modern investigation may be gained 

 from the fact that the nucleus of each of the two daughter- 

 cells which result from the first division of the fertilised 

 egg-cell is made up of chromatin contributions half 

 maternal and half paternal. This equal partition has 

 been followed for a number of successive divisions, the 

 actual demonstration of this being rendered possible in 

 some cases by a visible difference between the maternal 

 and paternal chromosomes. 



Five chief things happen when an animal ovum is fer- 

 tilised by a sperm. (1) There is a mingling of two inheri- 

 tances, of the maternal and paternal germ-plasms. (2) 

 There is a restoration of the number of chromosomes to 

 the normal. As the ripe ovum has had its number 

 reduced to one half of the normal, and as the same is 

 true of the ripe spermatozoon, the union of the two 

 must bring back the normal number, which is usually 

 adhered to in all the cells of the offspring. (3) The sper- 

 matozoon brings into the ovum a minute body known 

 as the centrosome, which plays an important part in the 

 subsequent division or segmentation. (4) When a 

 spermatozoon enters the ovum, there is a very rapid physi- 

 cal change in the periphery. The ovum becomes non- 

 receptive to other spermatozoa. The way is " blocked," 

 and this usually prevents multiple fertilisation, which is 

 one of the causes of abnormal development. (5) There 

 is some stimulus to the egg to divide or the removal of 

 some embargo which has kept the egg from dividing. 

 A very remarkable fact, established by Loeb, Delage, 

 and others, is that this part of fertilisation the setting 



