FROM SIMPLE CELL TO COMPLEX ANIMAL 43 



by the reduction division, one-half of the natural number of 

 chromosomes in the species. There is an increasing body of 

 evidence for the belief that the chromosomes chiefly bear the 

 hereditary characters from oiie cell to another and hence from 

 one generation to another. When the nucleus of the sperm 

 unites with the nucleus of the egg in fertilization one-half of 

 the chromosomes in the new nucleus thus comes from each of 

 the parents. Fertilization may be said then to do two things: 

 (a) it unites the substances carrying hereditary qualities from 

 two parents (usually) ; and (6) starts the development of the 

 egg (Figs. II, 12). 



2. Cleavage, and the Segregation oJGerm Cells. — The fertilized 

 egg soon begins to divide {cleavage). The resulting cells may be 

 much alike or may be very different in size and contents, from the 

 very beginning. (See Fig. 13.) Most of these cells enter directly 

 into the making of the adult body by the differentiation into 

 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 very 

 early put aside, which do not take direct part in the develop- 

 ment of the tissues of the body. By tracing the history of 

 these cells through the various stages of embryonic develop- 

 ment 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. 11 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. — 

 Froin 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 because of the hereditary qualities which 

 they carry are differentiating to form the tissues and organs 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, hut not differentiating. 

 During whatever time is necessary for the organism to develop 

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

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



