. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Preservation of Species 449 one should first of all distinguish the essentially "immortal germplasm" (Weismann) that bridges the generations, from the accessory structures which minister thereto, but are destined to perish with the life of the indi- vidual of which they form a part. III. THE ESSENTIAL REPRODUCTIVE CELLS Germplasm, the essential material concerned in reproduction, consists of sperm cells and eggs in sexual animals. 1. Sperm Ce
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Preservation of Species 449 one should first of all distinguish the essentially "immortal germplasm" (Weismann) that bridges the generations, from the accessory structures which minister thereto, but are destined to perish with the life of the indi- vidual of which they form a part. III. THE ESSENTIAL REPRODUCTIVE CELLS Germplasm, the essential material concerned in reproduction, consists of sperm cells and eggs in sexual animals. 1. Sperm Cells The detachable germinal units de- rived from the male individual are sperm cells. They are frequently called spermatozoa (sperma, seed; zobn, ani- mal), an awkward and misleading word, embalming the historical fact that when these cells were first discov- ered by early microscopists, they were thought to be tiny independent parasitic animals. A sperm cell in order to produce a new individual must always join forces with an egg cell. It can never become an individual unassisted. Such union, however, is not indispensable in the case of the egg cell, which among cer- tain invertebrates may develop parthen- ogenetically, that is, without the assist- ance of a male sperm cell. The sperm cells of different species have a chemical specificity for the eggs of their own kind, and animals do not bastardize under ordinary conditions even though their germ cells may have free access to each other, as in sea-water for example, where a variety of different kinds of eggs and sperm are pres- ent. If this were not true, untold confusion would result. The motility which enables the active sperm to seek out the compara- tively stationary egg is accomplished among vertebrates by the development. C E T Fig. 378. Diagrams of the develop- ment of spermatozoa, , anterior centrosome; axial filament; , connecting pieces; , chief piece; , galea capitis; n., nucleus; nk., neck; p.,
Size: 1374px × 1818px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte
