128 Darwin, and after Darwin. 



purpose of effecting fertilization. This spermatozoon, 

 as soon as it enters the periphery of the yolk, or cell- 

 substance proper, sets up a series of remarkable 

 phenomena. First, its own head rapidly increases in 

 size, and takes on the appearance of a cell-nucleus : this 

 is called the male pronucleus. At the same time its 

 tail begins to disappear, and the enlarged head proceeds 

 to make its way directly towards the nucleus of the 

 ovum which, as before stated, is now called the female 

 pronucleus. The htter in its turn moves towards the 

 former, and when the two meet they fuse into one 

 mass, forming a new nucleus. Before the two actually 

 meet, the spermatozoon has lost its tail altogether ; 

 and it is noteworthy that during its passage through 

 the protoplasmic cell-contents of the ovum, it appears 

 to exercise upon this protoplasm an attractive in- 

 fluence ; for the granules of the latter in its vicinity 

 dispose themselves around it in radiating lines. All 

 these various phenomena are depicted in the above 

 wood-cuts. (Figs. 34, 35.) 



Fertilization having been thus effected by fusion of 

 the male and female pronuclei into a single (or new) 

 nucleus, this latter body proceeds to exhibit compli- 

 cated processes of karyokinesis. which, as before 

 shown, are preliminary to nuclear division in the case 

 of egg-cells. Indeed the karyokinetic process may 

 begin in both the pronuclei before their junction is 

 effected ; and, even when their junction is effected, 

 it does not appear that complete fusion of the so- 

 called chromatin elements of the two pronuclei takes 

 place. For the purpose of explaining what this 

 means, and still more for the purpose of giving a 

 general idea of the karyokinetic processes as a whole. 



