. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 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 (, the stem-cell line) is destined to und
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 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 (, 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 (, 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 ST. of (D), the chromosomes are intact. 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 plas
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