SUMMARY OF CURRENT RESEARCHES. 
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evidence in the insectivorous Tupaia javartica. Sections of the ovum of 
this mammal show that during the early stages of cleavage there is no 
sensible difference in the size of the cells then arising ; as early, 
however, as the solid morula stage there is an unmistakable outer layer 
and inner core of cells. When there are more than a dozen of the latter 
a cavity arises, the outer layer becomes the wall of this early monodermic 
phase, and the inner core is massed together almost simultaneously. 
The inner core differentiates into a layer of flattened cells and a knob of 
more cubic ones ; the former arrange themselves into the inner wall of 
what then becomes a didermic blastocyst. The latter at the outset 
form a local thickening of this inner layer. The outer layer is what 
the author has called the trophoblast, the inner layer is the hypoblast. 
Prof. Hubrecht describes further stages in the development of this 
didermic blastocyst, and comes to the conclusion that the phenomena 
observed by him leave no doubt that the wall of the transitory mono- 
dermic stage is epidermic in nature and not hypoblastic as Robinson 
would have it. The author believes that in the formation of the 
mammalian blastocysts, ceenogenetic processes play a prominent part. 
It is by a precocious segregation that cell matter of epiblastic and of 
hypoblastic ancestry is arranged into a two-layered vesicle, whilst the 
really formative matter out of which the embryo would be built is yet 
quiescent in the embryonic knob. If we desire to have a clear con- 
ception of the real nature of the mammalian blastocyst we must acknow- 
ledge that the holoblastic segmentation of the mammalian ovum is 
something totally different from the holoblastic segmentation of very 
many Invertebrates and of Amphioxus. It is all the more misleading 
because it appears to be a paleogenetic feature. 
Attachment of Mammalian Embryo to the Walls of the Uterus.*— 
Mr. R. Assheton has made an investigation of the causes which lead to 
the attachment of the mammalian embryo to the walls of the uterus. 
He finds that the blastodermic vesicle of the Rabbit becomes first attached 
to the walls of the uterus by its lower pole. He regards this attachment 
as the result of a mechanical pressure of certain spots, or knobs, of 
thickened epiblast of the blastodermic vesicle upon the epithelium of 
the uterus. This pressure is hydrostatic within the vesicle and results in 
the uterine epithelium being pierced, when the knobs of epiblast become 
imbedded in the connective tissue below. He regards these knobs of 
epiblast as being the direct result of a destruction of the equilibrium 
between the rate of increase of the hydrostatic pressure within the 
blastodermic vesicle and the rate of growth of the cellular wall of the 
vesicle. The primary cause of “ inversion ” is the fact that the embryonic 
area is at one time a region of less activity, and is surrounded by a zone 
of greater activity, in connection with the future formation of the 
placenta. Inversion is determined by the heaping up of cells at an 
early stage by this zone of greater activity, whereby the embryonic area 
is forced inwards. Inversion is prevented by such causes as impede the 
heaping up of tissues around the embryonic area. Foremost amongst 
such causes is the presence of an investing coat, and the thicker and 
more lasting the coats are, the more marked are the intrinsic characters 
* Quart. Journ. Micr. Sci., xxxvii. (1894) pp. 173-90 (1 pi.). 
