42 THE EVOLUTION OF MAN 



EARLY PRENATAL DEVELOPMENT 



Reproduction is started by the union of the germinal cells 

 of the two sexes (Fig. n). These cells are differentiated in 

 development before the sixth week of prenatal life and set 

 aside in special organs, the gonads, for the perpetuation of 

 the race. Even at this early period the sex of the germinal 

 cells of the embryo is structurally indicated. The female cell, 

 or ovum, is spheroidal in shape and although it is one of the 

 largest cells of the body yet it is so small as to be hardly 

 visible to the naked eye. The male cell, or sperm, is very 

 much smaller even than the ovum and is elongated and 

 specialized for active motility. Both the sperm cells and the 

 ova during their development go through a series of changes 

 which result in the reduction to one half of the chromatin 

 material contained in the nucleus. This is an important fact 

 as the chromatin is regarded as the carrier of the hereditary 

 qualities, and the fusion of these two cells at the time of fertili- 

 zation, each with one half the normal amount of chromatin, 

 produces again a complete cell with the typical amount of 

 chromatin and having an equal proportion of maternal and 

 paternal hereditary substance. As a result of fertilization the 

 egg cell has the power of almost indefinite multiplication and 

 the still more marvelous power of differentiation so that its 

 descendants are not all alike, but some form nerve cells, others 

 gland cells, still others muscle cells, et cetera. This differentia- 

 tion in structure is accompanied also by a corresponding func- 

 tional differentiation. It may be possible to explain many of 

 the processes of life on the mechanistic, or physico-chemical 

 basis, but it is difficult at present to explain reproduction on 

 this theory. 



After fertilization, the repeated multiplication of the egg 

 cell quickly produces a solid spherical mass of similar cells, 

 still almost microscopic in size, called the morula. Some of the 

 cells soon become massed on one side and in this region two 



