ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
317 
Development of the Germinal Layers in Sorex.* — Prof. A. A. W. 
Ilubrecht, in the second of his studies in mammalian embryology, de- 
scribes the formation of the germinal layers in the shrew, and discusses 
the general problems connected with the subject. 
The earliest stage of which sections were made exhibits a distinct 
zona pellucida, internally clothed by a layer of flattened cells — the 
trophoblast, while at one spot there is an agglomeration of 50-60 larger 
cells — the material for the embryonic epiblast and for the hypoblast. 
The segmentation cavity beneath the polar thickening is distinct, though 
not yet very spacious ; after the development of the coenogenetic hypo- 
blast, it becomes the cavity of the yolk-sac. At this stage the embryos 
were still free in the uterus. 
The blastocyst widens, its cells are stretched, and the zona becomes 
thinner. In the last phases of the didermic stage, just before the appear- 
ance of a mesoblast and a gastrula ridge or primitive streak, the zona 
reaches its limit of tenuity. After that it disappears and the trophoblast 
is attached to the uterine tissue. From the embryonic knob hypoblast 
cells originate, which gradually form a continuous layer within the 
trophoblast. It seems likely that the trophoblast extends as a thin layer 
above the embryonic knob. After the migration of hypoblast cells from 
the embryonic knob, the patch of epiblast which remains may be termed 
the embryonic shield. 
The hypoblast forms a complete and closed sac, clothing the entire 
inner surface of the trophoblast. Just below the anterior end of the 
embryonic shield the hypoblast undergoes an important modification, 
forming a patch in which a differentiation occurs, which ultimately leads 
to the formation of the notochord and the lateral mesoblast plates. As 
part of this patch will develope into the anterior portion of the notochord, 
it is called the protochordal plate. It is remarkable that this should 
precede the first hints of a gastrula ridge. 
Towards the origin and further development of the middle layer in 
Sorex vulgaris three distinct sources contribute. These are : — (1) The 
protochordal plate ; (2) the gastrula-ridge and its median prolongation 
forwards — the protochordal wedge which advances between the epiblast 
and the hypoblast ; (3) an annular zone of hypoblast situated just out- 
side the limits of the embryonic shield, and thus inclosing — but at the 
outset independent of — the protochordal plate. But soon the mesoblast 
becomes a confluent layer, and grows by the division of its own cells. 
In his theoretical considerations on the gastrulation of the Mammalia, 
Prof. Hubrecht emphasizes the principle of precocious segregation as 
applied to part of the hypoblast. A didermic stage of the blastocyst is 
inaugurated before the actual process of gastrulation has set in. But 
another portion of the hypoblast arises in more palingenetic fashion, 
namely, in the gastrula ridge. An explanation is given of the manner 
in which the union of the palingenetic and coenogenetic hypoblast comes 
about. 
In explanation of the persistence of the yolk-sac in Didelphia and 
Monodelphia, Prof. Hubrecht notes that, for a satisfactory working of 
the new nutritive arrangements of the embryo, “it is undoubtedly of 
the utmost importance that the surface of the area vasculosa should be 
* Quart. Jouru. Micr. Sci., xxxi. (1890) pp. 499-562 (7 pis.). 
1891. 
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