FROM SIMPLE CELL TO COMPLEX ANIMAL 43 



entoderm and ectoderm, and later into nerve cells, muscle cells, 

 gland cells, and the like. It has been clearly shown, however, 

 for some species, that certain undifferentiated cells are sooner or 

 later put aside, which do not take direct part in the development 

 of the tissues of the body. By tracing the history of these cells 

 through the various stages of embryonic development it is found 

 that these are primordial or ancestral germ cells and that they 

 ultimately produce the sperm and egg cells with which we are 

 familiar. (See Figs, n and 12.) It is probable that something 

 similar will be found to be true of animals generally. 



3 . The Parallel Development of the Body and the Germ Cells. 

 From this time on two quite different things are happening in 

 every normal body as it develops: (a) the body cells are multi- 

 plying, growing, and differentiating to form the tissues and or- 

 gans of the animal; and (6) the primordial germ cells, which 

 have come from the same egg, and are cousins not descendants 

 of the body cells are dividing and growing but not differentiating. 

 During whatever time is necessary for the organism to develop 

 its body tissues and organs, the primordial germ cells are rinding 

 the permanent place they are to occupy in the body and are, 

 in cooperation with certain of the body cells, forming sex glands, 

 testes and ovaries. 



4. Sexual Maturity and the Perfection of the Ova and Sperm. 

 In this parallel development of the germ cells and the body cells 

 it is certain that they modify each other profoundly. The 

 germ cells depend for their supply of food and oxygen, etc., upon 

 the activities of the body cells. Undoubtedly the wastes of the 

 body cells also influence the germ cells. However, it seems that 

 the presence and products of the germ cells even more modify 

 the growth of the body. This is especially true as the germ cells 

 and glands approach maturity. In a number of species it has 

 been shown that the development of certain parts of the body 

 is very much changed if the maturing sex glands are removed 

 early in life. In general it is believed that the differences in the 

 bodies and the instincts of the male and female of a species are 

 largely due to the effect of the development of sperm and ova 

 within the bodies. 



In the last steps whereby the spermatogonia produce sperm 



