ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
321 
According to Roux, the regenerative mechanism is set at work mainly 
by the absence of the normal neighbour-influences, and the process 
spreads from the area of defect to the other cells. JIc recognizes two 
modes of development : the normal or direct, in which there is a typical 
system of definitely determined processes ; and the indirector atypical, in 
which the post-generative processes are at work — when, for instance, the 
half-embryo of a frog forms a whole embryo. He goes the length of 
speaking of the idioplasm of direct development, and the idioplasm of 
indirect development, the latter being awakened by the now well-known 
methods of artificial embryology. 
The main antithesis is obvious. Roux and Weismann believe in the 
specific character of the early blastomeres ; Driesch and Hertwig believe 
in their mutual equivalence. When a part of a segmented ovum gives 
rise to a normal whole, the latter say the fact proves the mutual equiva- 
lence of the early blastomeres ; the former say that it illustrates regene- 
rative and post-generative processes. 
Placentation of the Shrew.* — Prof. A. A. W. Hubrecht devotes the 
third of his ‘ Studies in Mammalian Embryology ’ to this Insectivore. 
He finds that the placentation of the shrew is brought about by sets of 
processes in the maternal tissue and the blastocyst that are at first in- 
dependent of one another ; later on, when the blastocyst has come to 
adhere against the maternal tissue, they are closely related ; still later 
the mother contributes nothing but blood. 
The maternal processes are unexpected and somewhat peculiarly 
shaped local distensions of the wall of the* uterus, with changes in the 
distribution of glandular tissue, &c., in this wall ; there are, also, con- 
siderable local proliferations of maternal uterine epithelium. 
The embryonic processes are local changes in the outer wall of the 
blastocyst, with special development of certain portions of the tropho- 
blast which finally constitute a syncytium, in which the allantois-villi and 
the embryonic blood are in the closest contact with maternal blood ; the 
latter circulates in spaces of embryonic tissue without any endothelial 
lining. 
The result of the changes in the tissue of the uterine wall is the 
formation of a concave bell-shaped surface, opposite the mesometrium, 
on which there open a number of newly formed epithelial crypts, be- 
tween which there are some glandular openings. Lateral cushion-shaped 
surfaces are also formed, against which the blastocyst first adheres by 
means of a zonary strip. Against this strip the blood-vessels of the area 
vasculosa on the yolk-sac spread out. In the embryonic syncytium 
already mentioned there may be distinguished a zonary syncytium in the 
region of the area vasculosa ; this owes its origin to that portion of the 
surface of the blastocyst that the author calls the “ omphaloidean tropho- 
blast.” Opposite the mesometrium there is a bell-shaped syncytium, 
which owes its origin to that part of the outer wall of the blastocyst 
which expands simultaneously with the formation of the amnion, and 
which the author calls the “ allantoidean trophoblast.” 
In the course of farther changes this trophoblast is applied against 
the concave maternal surface, and sends knob-like projections into the 
* Quart. Journ. Micr. Sci., xxxv. (1894) pp. 481-537 (9 pis.). 
1894 
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