. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 2/4 BASHFORD DEAN. however, a rate of development proportionate with that of known egg-depositing elasmobranchs I infer that an embryo of five inches can hardly be younger than nine months (Fig. 7). An estimate of a total incubation of twelve months in this species would hardly be excessive. Fertilisation.—Fertilization is shark-like. Polyspermy occurs and a similar number of merocyte nuclei; the conjugation of the pronuclei occurs at a similar niveau in the germinal area, the male pronucleus passing through the germinal pro
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 2/4 BASHFORD DEAN. however, a rate of development proportionate with that of known egg-depositing elasmobranchs I infer that an embryo of five inches can hardly be younger than nine months (Fig. 7). An estimate of a total incubation of twelve months in this species would hardly be excessive. Fertilisation.—Fertilization is shark-like. Polyspermy occurs and a similar number of merocyte nuclei; the conjugation of the pronuclei occurs at a similar niveau in the germinal area, the male pronucleus passing through the germinal protoplasm and then approaching the female pronucleus from a lower plane. Also similar are the location and behavior of the merocyte nuclei during early cleavages. Different from shark, however, is the longer duration of the period of the entrance of the sperm (a newly-entered spermatozoon being present in a preparation showing fusion of pronuclei), and the clearness with which sperm paths are to be noted. Of the latter the surface pits can be seen under low powers. The germinal area is also notably deeper than in the shark. Segmentation.—As in Torpedo (Riickert) and certain other elasmobranchs the appearance of cleavages at the surface of the germinal area is retarded, the first surface furrow appearing about the time of the third or fourth nuclear division. In connection with the furrows at their deeper rim are vacuoles (as in Fig. 8. FIG. 8. Early segmentation. Section through middle of germinal area. At A a fissure-like vacuole becomes continuous with an intercellular space. / about 30. at A) which pass into the yolk, and suggest a former condition of deeper cleavage. This inference that the fissure-like vacuoles are to be interpreted as potential but semi-suppressed cleavage spaces is well borne out by the structures of a later stage (Fig. 9) in which the vacuoles are shown to be patently continuous with intercellular spaces, and in which nuclei occur in the underlying ge
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
