The early development of the marsupialia, with special reference to the native cat (dasyurus viverrinus) / by J.P. Hill.

  • Hill, J. P.
Date:
[1910?]
Catalogue details

Licence: In copyright

Credit: The early development of the marsupialia, with special reference to the native cat (dasyurus viverrinus) / by J.P. Hill. Source: Wellcome Collection.

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    enormous tliickness of the albumen layer], iu which the four equal-sized blastomeres are radially arranged round a cleavage cavity and are conical in form, their upper ends being more pointed, their lower ends thicker and richer in yolk-material. The nucleus of each is excentric, being situated nearer the upper pole. [This description is applicable word for word to the4-celled stage of Dasyurus.] An 8-celled stage (fig. 6) is next described, seven of the blastomeres being equal in size and one being smaller. They are arranged somewhat irregularly iu two circles. [This stage I regard as abnormal both in respect of the arrangement of the blastomeres and the occurrence of irregularity amongst them.] Selenka (p. 119) thought it probable that the third cleavage planes cut the first two at right angles and divided each of the first four blastomeres into a smaller ectodermal cell and a larger more granular entodermal, but states that he was unable to establish this owing to the opacity of the albumen-layer. [My observations show that it is the fourth cleavage in Dasyurus, not the third, which is equatorial, un- equal, and qualitative, and that even then the cells formed are not ectodermal and entodermal insignificance. The albumen is normally never opaque.] A 20-celled stage is mentioned, but not described, since it suffered in preparation. It is said to have a large entoderm cell in the cleavage cavity. [A statement of very doubtful value, since the blastomeres were admittedly pressed together and probably displaced by the shrunken egg-membranes.] The next stage described is a spherical “gastrula^’ (Taf. xvii, figs. 7, 8), composed of fox’ty-two cells with an open blasto- pore^’ at the vegetative pole, a smaller opening at the animal pole, and a large “ ur-entoderm ” cell in the cleavage-cavity, just inside the blastopore.” The wall of the gastrula” consists of cells graduated in size ; those in the region of the blastopore are the largest and richest in deutoplasm, those at the opposite pole are the smallest and most transparent. [This is a very characteristic stage in the formation of the blastocyst, with which I am quite familar in Dasyurus. Selenka’s speci-