116 



THE DEVELOPMENT OF THE GAMETES OR SEX CELLS 



GEHM- CELL 



LINE I ECTO DER M I ENTODE R M V STOMODAEAL 

 MESODERM MESODERM MESODERM 



ECTODERM 



Fig. 61 Diagrammatic representation of the process of chromatin diminution in the 

 nematode worm, Ascaris equorum {A. megalocephala), and of the "Keimbahn" (in 

 black, E). One daughter cell shown by the four black dots of each division of the germ- 

 cell line (i.e., the stem-cell line) is destined to undergo chromatin diminution up to the 

 16-cell stage. At the 16-cell stage, the germ-cell line ceases to be a stem cell (e.g., P,), 

 and in the future gives origin only to sperm cells (E). (A-D, copied from King and 

 Beams ('38); E, greatly modified from Diirkin ('32).) 



Animal pole of the cleaving egg (A) is toward the top of the page. (B) Metaphase 

 conditions of the second cleavage. Observe the differences in the cleavage planes of the 

 prosomatic cell, S,, and that of the stem cell, P,. (C) Anaphase of the second cleavage 

 of S,. Observe that the ends of the chromosomes in this cleaving cell are left behind on 

 the spindle. (D) It is to be noted that the ends of the chromosomes are not included 

 in the reforming nuclei of the two daughter cells of S,, thus effecting a diminution of 

 the chromatin substance. In P,, P,, and E.M. ST. of (D), the chromosomes are intact. 

 E.M. ST. = second prosomatic cell. MST = mesoderm-stomodaeal cell. 



A second contributory concept to the germ-cell (germ-plasm) theory was 

 made by Nussbaum, 1880; Boveri, 1892, '10, a and b, and others. These 

 investigators emphasized the possibility that a germinal cytoplasm also is 

 important in establishing the germ plasm of the individual. A considerable 

 body of observational and experimental evidence derived from embryological 

 studies substantiates this suggestion. Consequently, the modern view of the 

 germ cell (germ plasm) embodies the concept that the germ cell is composed 



