INTERNAL CAUSES OF VARIATION 161 



This view of the case is borne out by the facts of fecundation 

 or fertilization of the ovum, which may be briefly described as 

 follows : l 



The ovum. This is the finished product of the sexual cells of 

 the mother parent, and consists of a nucleus with its character- 

 istic chromatin granules surrounded by a comparatively large 

 mass of cytoplasm. Its equivalent in plants is the ovule. 



The spermatozoon. This is the characteristic product of the 

 sexual cells of the male in animals, and is the functional equiva- 

 lent of the pollen grain and the spermatozoid of plants. It is 

 in all cases vastly smaller than the corresponding ovum, being 

 almost destitute of cytoplasm. The characteristic elements of 

 the ovum are its nucleus and the cytoplasm, while the character- 

 istic elements of the spermatozoon are its nucleus, borne in the 

 "head," and a centrosome, generally carried in the " middle 

 piece." The tail, formed from the small amount of cytoplasm, 

 seems to have no function beyond providing motile power, and is 

 absent in the pollen of higher plants. 



Fertilization. Both the ovum and the sperm cell have arisen 

 in their respective organs by the method of cell division, display- 

 ing in the process the ordinary phenomena of mitosis. 2 But 

 both have reached the end of their powers of self-division, and if 

 left alone they will be thrown off from their respective points of 

 origin to wither and die. 



If, however, they are brought near together, mutual attraction 

 ensues, the spermatozoon (or other sperm cell) enters the ovum, 

 the nuclei approach each other and fuse, the centrosome divides, 

 an amphiaster is formed, and cell division ensues. The ovum is 

 now fertilized, segmentation proceeds, and a new individual is 

 established in an independent existence. 



The new individual is thus the possessor of actual living mat- 

 ter (physiological units) derived from both parents, and thus 

 inherits literally the substance of both, having come into direct 

 possession of material identical with the living matter of both 

 parents. 



1 For a fuller discussion of this subject, see Wilson, The Cell, pp. 178-231. 



2 For a brief statement of what is involved in maturation, see the next 

 section. 



