The cell in development and inheritance . portance of this dis-covery ; for although it stands at present an almost isolated case, yetit gives us, as I believe, the key to a true theory of differentiationdevelopment,^ and may in the end prove the means of explainingmany phenomena that are now among the unsolved riddles of the cell. Hacker (95) has shown that the nuclear changes in the stem-cells and primordial eggs of Cyclops show some analogy to those ofAscaris, though no casting out of chromatin occurs. The nuclei arevery large and rich in chromatin as compared with the somatic cells,and the
The cell in development and inheritance . portance of this dis-covery ; for although it stands at present an almost isolated case, yetit gives us, as I believe, the key to a true theory of differentiationdevelopment,^ and may in the end prove the means of explainingmany phenomena that are now among the unsolved riddles of the cell. Hacker (95) has shown that the nuclear changes in the stem-cells and primordial eggs of Cyclops show some analogy to those ofAscaris, though no casting out of chromatin occurs. The nuclei arevery large and rich in chromatin as compared with the somatic cells,and the number of chromosomes, though not precisely determined, 1 Boveri, 91, p. 437. ^ 0^ P- 426. ORIGIN OF THE GERM-CELLS 149 is less than in the somatic cells (Fig. 74). Vom Rath, workingin the same direction, believes that in the salamander also thenumber of chromosomes in the early progenitors of the germ-cellsis one-half that characteristic of the somatic cells.^ In both thesecases, the chromosomes are doubtless bivalent, representing two. Fig. 74. — Primordial germ-cells in Cyclops. [HACKER.] A. Young embryo, showing stem-cell (sf). B. The stem-cell has divided into two, givingrise to the primordial germ-cell {g). C. Later stage, in section; the primordial germ-cell hasmigrated into the interior and divided into two; two groups of chromosomes in each. chromosomes joined together. In Ascaris, in like manner, each ofthe two chromosomes of the stem-cell or primordial germ-cells isprobably plurivalent, and represents a combination of several unitsof a lower order which separate during the segmentation of thethread when the somatic mitosis occurs. 1 C/. p. 256, Chapter V. 150 THE GERM-CELLS D. Growth and Differentiation of the Germ-cells I. The Ovum {a) Growth and Nutrition. — Aside from the transformations ofthe nucleus, which are considered elsewhere, the story of the ova-rian history of the is largely a record of the changes involved innutrition and the storage of materi
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