in the mammalian ovum, and the
whole course of segmentation in the ovum of the rabbit from the 2-celled
to the morula stage was carefully described and figured by Barry[269] in
1839. C. Vogt[270] in 1842 described segmentation in _Coregonus_ and
_Alytes_. The discovery of segmentation in the ovum of birds was not
made until 1847, by Bergmann,[271] confirmed independently by Coste[272]
in 1850. By 1848 segmentation had been noted in _Hydra_ and various
hydroids, in acalephs, in starfish, polyzoa, nematodes, rotifers,
leeches, oligochaetes, polychaetes, in most groups of molluscs and
arthropods, and in all the vertebrate classes.[273]

The process was at first held to be merely one of yolk-division, or
_Dotterfurchung_, and its details were by most interpreted in the light
of the Schleiden-Schwann theory of cell-formation.

The first steps towards a truer conception of the process seem to have
been taken by Bergmann, who in 1841[274] called attention to the presence
of nuclei in the segmentation-spheres of the frog's egg, and by Bagge in
the same year, who observed that division of the nuclei preceded the
multiplication of the segmentation spheres.[275] He considered the nuclei
to be anucleate cells, and the same view was taken by Koelliker in
1843.[276] Next year, however, in his classical paper on Cephalopod
development[277] Koelliker came to the opinion that they were really
nuclei. He showed that segmentation was brought about by cell-division,
that between "total" and "partial" segmentation there was a difference
of degree and not of kind, and that the cells of the body were formed by
division of the segmentation spheres. He held, however, that the nuclei
multiplied endogenously and not by division. The division of nuclei was
observed by Coste in 1846.[278] Leydig in 1848[279] took the necessary step
in advance and maintained that the nuclei as well as the cells increased
always by division. He was supported by Remak, who in a paper of
1852,[280] and more fully in his monumental _Untersuchungen ueber die
Entwickelung der Wirbelthiere_ (Berlin, 1850-55), proved that in the
frog's egg at least segmentation was a simple process of cell-division,
initiated always by division of the nucleus.[281]

One point Remak left undecided--the fate of the _Keimblaeschen_ or
egg-nucleus. It was generally held, even so late as the 'fifties, that
the egg-nucleus disappeared just before segmentation began--Bischoff
clung to this beli