356 MINOT. [Vou. IL. 
specimens, is continued around. It is probable that when 
embryos of nine days are removed, that a portion of the ecto- 
derm remains attached to the uterine wall, and consequently the 
inferior portion of the vesicle is without any ectoderm. Being 
unaware of such a possibility, van Beneden and Julin have 
perhaps represented the single layer left as ectoderm on 
account of the theoretical necessity of an ectodermal cover- 
ing on the external or apparently external surface of the ovum. 
The question, therefore, is to be posed: Have not observers 
found two layers up to a certain limit beyond the vena termin- 
alis, and only one layer over the remaining inferior portion of 
the embryonic vesicle, and assumed the single layer to be ecto- 
derm, whereas it is entoderm, and the true ectoderm is left upon 
the uterus, to which it is indissolubly attached? The view I 
advocate brings the further question whether a portion of the 
embryonic ectoderm disappears by being involved in the resorp- 
tion of the ob-placental uterine epithelium. This I think is not 
the case. The intimate adherence of the extra-embryonic por- 
tions of the germ layers to the uterine walls has been carefully 
recorded by Bischoff, Extwzckelungs gesch. Kantnchens, p. 131, 
“Vom dem Umkreise der Vena terminalis an ist das Ei [of ten 
days] in die in dieser Lage, etc... . und von hier an sind auch alle 
Eihaute so innig unter einander und durch den Uterus vereinigt, 
dass es nicht gelingt sie zu lésen.” 
The attachment of the embryo takes place as described by 
van Beneden and Julin, p. 402, 403, by an area of thickened ecto- 
derm; the general arrangement is well shown in Cut 1, while 
the fitting together of the foetal and maternal surface is better 
illustrated by Fig. 7, Pl. XXVII. The foetal mesoderm does not 
participate even indirectly in this attachment, but runs along 
free from the outer germ layer. The ectoderm, as it nears its 
attachment (see Fig. 7), gradually thickens. Just where it joins 
the uterine surface there are several large cells with very large 
nuclei; appearances which are probably connected with the 
growth of the layer, for beyond the line of the large cells the 
ectoderm is very much thicker. Extremely distended nuclei 
also occur very strikingly in the developing supra-renal capsules, 
and are also there connected presumably with cell proliferation. 
If these suppositions are correct, there is a modified form of cell 
division characterized by dilatation of the nuclei and which 
Se 
