22 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
of the four-celled stage be completely isolated, each may give rise to a 
dwarf blastula, gastrula, and oval free-swimming embryo, of one-fourth 
the normal size. If the same stage fall into two pairs of cells, each pair 
forms an embryo of half the normal size. If the four blastomeres be 
imperfectly separated, three types of gastrulae arise : — double embryos, 
triple embryos (one twice the size of the other two), and quadruple 
embryos (each one-fourth of the normal size). It seems likely that the 
isolated blastomere of the eight-celled stage is incapable of producing 
a gastrula, and that this is due to qualitative rather than to quantitative 
limitations is suggested by the developmental vigour of these one-eighth 
embryos, and by the fact that under certain conditions (from two- or 
four-celled stages by fission or breakage of a blastomere) minute gastrulse 
are produced which are even less than one-eighth of the normal size. 
In the normal segmentation, the fourth cleavage is not strictly meri- 
dional and radial, as Hatschek described, but is either (1) bilaterally 
symmetrical with reference to the first cleavage-plane (and very like that 
of Ascidians), or rarely (2) approaches the radial form, or most rarely 
(3) is of the true spiral type found in Annelids and Molluscs. 
The cleavage of a completely isolated blastomere of the two-celled 
or four-celled stage is not a half-cleavage, but agrees essentially with 
that of a complete normal ovum. The development of the isolated unit 
is transformed from the beginning, and thus differs from that described 
by Roux for similar cases in the frog and by Driescli for the sea-urchin, 
for in both of these the development at first agrees with that of a normal 
embryo-half, and only later gives rise to a perfect embryo by a process 
which may be provisionally called “ regeneration.” 
Phytogeny of Mammalian Teeth.* — Dr. C. Rose notices that various 
investigators had hinted at his theory of the origin of Mammalian 
molars by fusion of simple conical teeth. Giebel (1856), Gaudry (1878), 
Magitot (1883), and Dybowsky (1889) are noted. He points out that 
the general idea is thus neither original to Kiikenthal nor to himself. 
In support of the coalescence theory he marshals numerous arguments, 
most of which are or have been noticed in this Journal in recording 
Rose’s concrete researches. 
History and Homologies of Human Molar Cusps.j — Prof. H. F. 
Osborn reviews the contributions of A. Fleischmann, J. Taeker, and 
C. Rose, and maintains the theory previously propounded by Cope and 
by himself. The primitive form of mammalian molar was a single cone, 
to which all the other cusps have been successively added. The proto- 
cone is invariably the anterior lateral (antero-external) cusp in the 
lower molars, and the anterior lingual (antero-internal) cusp in the 
upper molars. 
Beginning with a single-fanged conical reptilian tooth, such as 
persists in Cetacea, Osborn finds the first departure towards a develop- 
ment of lateral cusps in the Triassic Dromotherium , the second in the 
contemporary Microconodon , the third in the Jurassic Sjpalacotherium , 
the fourth in the Jurassic Ampliitherium, in which are seen the three 
cusps of the primitive triangle and the first cusp of the talon. In Miacis 
* Biol. Centralbl., xii. (1892) pp. 024-38. 
t Anat. Anzeig., vii. (1892) pp. 740-9 (3 figs.). 
