The observations of R. Hertwig[96] on the gemmation of _Podophrya gemmipara_, support my interpretation of the knobbed condition of the nuclei. Hertwig finds (p. 47) that
The horse-shoe shaped nucleus grows out into numerous anastomosing projections. Over the free ends of the projections little knobs appear on the surface of the body, into which the lengthening ends of the processes of the nucleus grow up. Here they bend themselves into a horse-shoe form. The newly-formed nucleus then separates from the original nucleus, and afterwards the bud containing it from the body.
Footnote 96: _Morphologisches Jahrbuch_, Bd. 1. pp. 46, 47.
From the peculiar arrangement of the net-work of lines of the yolk around these knobbed nuclei, it is reasonable to conclude that interchange of material between the protoplasm of the yolk and the nuclei is still taking place, even during the later periods.
These facts about the distribution in time of the cone-like bodies afford a strong presumptive evidence of a change in the manner of nuclear increase.
The last argument I propose urging on this head is derived from the bodies (Pl. 6, fig. 8_a_, _b_, _c_) which I have described as intermediate between the true cone-like bodies and typical nuclei. They appear to afford evidence of less and less of the matter of the nucleus streaming out into the yolk and of a large proportion of it becoming divided.
The conclusion to be derived from all these facts is that for Elasmobranchii in the earlier stages of segmentation, and during the formation of fresh segments, a partial solution of the old nucleus takes place, but all its constituents serve for the reconstruction of the fresh nuclei.
In later periods of development a still smaller part of the nucleus becomes dissolved, and the rest divides; but the two fresh nuclei are still derived from the two sources. After the close of segmentation the fresh nuclei are formed by a simple division of the older ones.
The appearance of the cone-like bodies in the yolk outside the germinal disc is a point of some interest. It demonstrates in a conclusive manner that whatever influence (if any) the nucleus may have in ordinary cases of cell division, yet it may undergo changes of a precisely similar character to those which it experiences during cell division, without exerting any influence on the surrounding protoplasm[97]. If the lobate nuclei are also nuclei undergoing division, we have in the egg of an Elasmobranch examples of all the known forms of nuclear increase unaccompanied by cell division.
