. Chimæroid fishes and their development. Fishes; Chimaeridae. 62 CHIM^ROID FISHES AND THEIR DEVELOPMENT. is opposed, on the other hand, by the conditions in the egt; of Cestracion, where the peripheral furrows, similar in general regards, are known to be continuous with those of true cleavage. The question, therefore, can not be answered finally until evidence is forthcoming to distinguish the kinds of nuclei present in the extra- embryonic yolk masses. Meanwhile, judging at least from the behavior of the nuclei in the circumgerminal ring, I think it is not at all improbable (cf. Gastru- lati
. Chimæroid fishes and their development. Fishes; Chimaeridae. 62 CHIM^ROID FISHES AND THEIR DEVELOPMENT. is opposed, on the other hand, by the conditions in the egt; of Cestracion, where the peripheral furrows, similar in general regards, are known to be continuous with those of true cleavage. The question, therefore, can not be answered finally until evidence is forthcoming to distinguish the kinds of nuclei present in the extra- embryonic yolk masses. Meanwhile, judging at least from the behavior of the nuclei in the circumgerminal ring, I think it is not at all improbable (cf. Gastru- lation) that in these masses both sperm and segmentation nuclei are present. Returning again to the development proceeding at the animal pole of the egg: We recall that in the sections figs. 53-56 there was shown a stage of late seg- mentation, or an early blastula, such, for example, as pictured in plate iv, fig. 27. In a slightly later stage (plate tv, fig. 28) an increased number of blastomeres are present, and there is still an indefinite condition in the periphery of the germ, blastomeres being continued irregularly over the ring-like circumgerminal zone. On the other hand, in plate iv, fig. 29, a stage is figured earlier than the Fig. 62.—Section ol blaslula. ,s<, Segmentation cavity. but showing a well-marked line of demarcation between the blastomeres and the cir- cumgerminal zone. It seems evident, accordingly, from this and similar instances, that considerable variation occurs as to the time at which the marginal relations of the germ are established. Thus in the stage first referred to (figs. 53-56) the circumgerminal zone was traversed b}' radial fissures and invaded by nests of cells; in a similar stage (plate iv, fig. 29, sectioned in fig. 62) the same region is solid and yolk-filled, forming a compact border to the germ. In contrasting these two stages one observes that, while they can differ little in point of age, judging from the number of blasto
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectfishes, bookyear1906
