396 A. A. W. HUBRECHT. 
PLATE XVII. 
Fires. 21—26.—Outlines taken with the camera of sections through very 
early embryos of Hrinaceus. All these six drawings are rigorously on the 
same scale of enlargement. 
Fig. 21 is taken through the earliest embryo in my possession. Figs. 
22—24 are three consecutive sections through another early embryo 
(Mus. Utr. Cat. n* Erin. 48a). Figs. 25 and 26 are again two 
different embryos (Mus. Utr. Cat. n°. Erin. 48 4? and c), but were both 
taken simultaneously from the same mother as Figs. 22—24. In the 
last-named figures the hypoblast cells have increased in number. In 
Figs. 23 and 24 they commence to expand, enclosing a central cavity 
between them. In Figs. 22 and 23 the very earliest indication of the 
polar knob of the trophoblast is visible, viz. as a few cells (Zp.) 
(in the figure they are distinguished by somewhat darker nuclei) 
projecting more or Jess into the segmentation cavity. In Fig. 25 the 
polar knob is much more distinct, the hypoblast a closed vesicle without 
as yet any difference between the constituent cells. In Fig. 26 both 
the trophoblast and the polar knob (ep.) have again increased, whereas 
the hypoblast clearly differentiates into an embryonic thickened and a 
peripheral flattened portion. 
Fie. 27 (cf. Fig. 16) is meant to show the intimate blending between the 
villiferous trophoblast (Z7.) of this stage with the surrounding maternal tissue 
(for better histological detail cf. Figs. 40—54). The epiblast of the germinal 
area is indicated by #. The hypoblastic vesicle (Ay.) is evidently contracted 
and folded (a very different phenomenon from what is noted in Figs. 22—25). 
Maternal blood penetrates into the lacune (sp.) that are contiguous with the 
trophoblast, and that are immediately derived from the same spaces (sp.) of 
Figs. 7 and 8. 
PLATES XVIII & XIX. 
The figures on these two plates are diagrammatic representations of the 
relation of the embryo to the uterine tissue and of the formation of the fcetal 
membranes. The gradual increase in size is made to correspond with the 
actual phenomenon; the enlargement is between four and five times. The light 
grey tint is altogether maternal tissue (the mesometrium pointing downwards) ; 
the darker grey is that mixture of modified maternal and of embryonic tissue 
which I have called the trophosphere. Furthermore, the trophoblast and 
epiblast have a black, the mesoblast an orange, the hypoblast a blue tint. The 
non-pregnant uterus is not here represented, but on Pl. XV, Fig. 1. Of the 
histological details represented on the following plates, the correspondence with 
the diagrammatic stages here figured is thus: Figs. 37 and 38 correspond to 
