. The germ-cell cycle in animals . Cells. 90 GERM-CELL CYCLE IN ANIMALS. Fig. 29. — Diagram to illustrate the phylogenetic shifting back of the origins of the germ cells in medusoids and hydroids. A composite picture. A, branch of a polyp-colony; P, polyp-head with mouth (tti) and tentacles ; >S^, stalk of the polyp; M, medusoid-bud with the bell ((?0 ; T, marginal tentacle; 7n, mouth; ilfs(, ma- nubrium ; GphK, a gonophore-bud; GH, gas- tric cavity; eki, ectoderm; ent, endoderm; st, supporting lamella. The germ cells (kz) arise in the medusoid in the ectoderm of the manubrium—first phyleti
. The germ-cell cycle in animals . Cells. 90 GERM-CELL CYCLE IN ANIMALS. Fig. 29. — Diagram to illustrate the phylogenetic shifting back of the origins of the germ cells in medusoids and hydroids. A composite picture. A, branch of a polyp-colony; P, polyp-head with mouth (tti) and tentacles ; >S^, stalk of the polyp; M, medusoid-bud with the bell ((?0 ; T, marginal tentacle; 7n, mouth; ilfs(, ma- nubrium ; GphK, a gonophore-bud; GH, gas- tric cavity; eki, ectoderm; ent, endoderm; st, supporting lamella. The germ cells (kz) arise in the medusoid in the ectoderm of the manubrium—first phyletio stage—where they also attain maturity. In the gonophore-bud (GphK) they arise in the ectoderm (fcz'), or further down in the stalk of the polyp at kz" — third phyletio stage — or in the ectoderm of the branch from which the polyp has arisen, at kz'" — fourth phyletic stage of the shunting of the originative area of the germ cells. In the last two cases the germ cells migrate until they reach their primitive place of origination in the medusoid, or in the corresponding layer of the medusoid gonophore, as may be more clearly seen in Fig. 30. (After Weismann, 1904-) of these cells in younger em- bryos than yet recorded, and might even dis- close charac- teristics which would enable us to trace the keimbahn in some species back into the early cleavage stages. In discussing the germ cells of coelenterates, it is necessary to refer to the work of Weis- mann who has added so much to our knowl- edge of this subject. Weis- mann's position may best be presented in his own words (The Evolution The- ory, Vol. I, pp. 413-415, 1904).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hegner, Robert William, 1880-1942. New York : The Macmillan Company
Size: 1223px × 2044px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjectcells, bookyear1914
