Chap. VI.] GENERAL EMBRYOLOGY. 89 



After this has taken place the ovum is ready for impregnation. 

 Here again we are forced to infer, from the results of investiga- 

 tions on other forms of life, the nature of the process of fertili- 

 sation. It would seem, however, that only one spermatozoon 

 enters the ovum. This spermatozoon is essentially a cell, and 

 its head contains the sperm-nucleus. When it enters the 

 ovum the sperm-nucleus forms the structure known as the 

 male pronucleus. The male pronucleus and female pronucleus 

 now approach each other, and finally fuse, the fused product 

 being known as the first segmentation nucleus. 



Upon this the process of segmentation sets in, and does not 

 cease until, by repeated cell-division, the fertilised ovum gives 

 rise to the adult organism. It is obvious that during this pro- 

 cess the cells must be nourished; (1) either by gradual absorption 

 of food-yolk j or (2) by the individual efforts of the developing 

 embryo ; or (3) by absorption of nutriment elaborated by the 

 mother. The frog, after the somewhat scanty stock of food- 

 yolk is absorbed, is hatched, and forthwith has to obtain food 

 for itself. The fowl undergoes a far larger proportion of its 

 development within the egg at the expense of the abundant 

 supply of food-yolk ; then it fends for itself, with some maternal 

 aid and advice. The rabbit is almost from the first supplied 

 with nutritive material by the mother, within whose body 

 embryonic development takes place ; and even after birth feeds 

 for some time on the mother's milk. 



The process of segmentation is essentially the division of the 

 primitive cell into two cells, each of these again into two, and 

 so on until the one has become a multitude. In each case of 

 cell-division there is also nucleus-division. And it has been 

 found that the reticulum of the nucleus undergoes a remarkable 

 series of changes constituting karyokinesis. 



Fig. 31, i. shows diagrammatically a cell in which the nucleus 

 is in the ordinary reticulum condition. In certain cases the 

 following changes have been shown to occur. When karyokinesis 

 sets in the capsule or limiting membrane of the nucleus tends to 

 disappear, and the protoplasm of the nucleus takes on a striated 

 appearance giving rise to a nuclear spindle, from the ends of 



