202 EVOLUTION, GENETICS, AND EUGENICS 



ends of the chromosomes and the disintegration into small chromatin 

 granules of the middle portions. The large ends of the chromosomes 

 are discarded into the cytoplasm and absorbed, while the smaller 

 granules are all that the descendants of this cell have for chromosomes. 

 It is significant that the progeny of such cells form only body cells, in 

 this case skin and nerve cells (Fig. 44, A). The other cell remains just 

 like the original zygote and is a stem cell or germ cell. This cell then 

 divides, and one of the daughter cells retains the full germinal charac- 

 ter, while the other breaks down its chromosomes as before and gives 

 rise to tissues that line the digestive tract (Fig. 44, B) Once more the 

 germ cell divides and one of its daughter cells undergoes chromosome 

 breakdown, its cell progeny forming chiefly such tissues as muscles, 

 blood vessels, and connective tissues (Fig. 44, D). From this point 

 on, the germ cells are definitely set apart and contribute no further to 

 the soma. They give rise to nothing but germ cells and ultimately to 

 gametes. The unbroken series of cells with intact chromosomes from 

 the zygote to gametes is the germ track, and the chromosomes of these 

 cells carry the so-called germ plasm. The rest of the embryo consti- 

 tutes the soma or somatoplasm. The relation of body to germ plasm 

 is well shown in the accompanying diagram (Fig. 45). 



A second example of a well-defined germ track is found in Miastor 

 (Fig. 46, A), in which a single definite germ cell {p.g.c.) is set aside at 

 the very first division of the zygote. The other cell divides repeatedly, 

 casting out parts of the chromosomes, as in Ascaris, and ultimately 

 giving rise to all the soma. Even in a fairly advanced embryo (Fig. 

 46, B) the large germ cells are clearly seen in a small group (oog 3 ). 

 These give rise ultimately to the gametes. 



While in vertebrates the germ track is ill defined and difficult to 

 follow, there seems to be no doubt that it exists. Some investigators, 

 however, claim that the original primordial germ cells disintegrate and 

 come to naught and that the gametes arise from a new lot of germ cells 

 that are derived afresh from generalized epithelium. If that be the 

 case, it seems fair to consider this epithelium as part of the germ track 

 and not as a differentiated part of the soma. At least it may be said 

 with confidence that in none of the higher mammals are germ cells ever 

 derived from specialized body cells such as muscle cells, nerve cells, or 

 gland cells. Our question as to the origin of the germ cells seems to 

 be answered. They are derived in an unbroken series from previous 

 germ cells by the process of mitosis. This is the basis of Weismann's 

 concept of the continuity of the germ plasm. 



