306 PROCEEDINGS OF THE AMERICAN ACADEMY. 



duce the dulermic blastocyst, to what origin are we to ascribe the 

 outer layer of " Deckzelleu " which I have described ? 



The last theory which I shall mention is that suggested by Robin- 

 son ('92). His studies on the embryos of the rat and the mouse have 

 led him to believe that the portion of the outer layer lying beyond 

 the germinal disk is entoderm, while the ectoderm is limited to the 

 outer layer of the germinal disk region. In the rat and the mouse 

 he does not find the ectoderm overgrowing the entoderm. However 

 correct this theory may be for the rat and the mouse, (my own work 

 on these animals leaves me still undecided on this point,) my investi- 

 gations on the pig show conclusively that the entodermal vesicle 

 becomes entirely surrounded by ectoderm. 



I can add little towards determining whether in the pig the extension 

 of the entoderm over the inner surface of the ectoderm takes place by 

 a growth proceeding from the margin of the entodermal portion of the 

 germinal disk, as in the rabbit, mole, etc., or whether this entodermal 

 mass of the germinal disk becomes a hollow vesicle, which reaches the 

 ectodermal wall by a multiplication and expansion of its cells, such 

 as would seem to take place in the hedgehog (Hubrecht '89, Figs. 

 7, 8, and 9, Plate XV.) and in the cat (Scliafer '76, Fig. 1, Plate X.). 

 The youngest embryo which I have described, from which Figs. 7 , 8, 

 and 9, Plate II., were made, showed, as I have already said, an area at 

 the end of the vesicle farthest from the germinal disk, where for several 

 sections no entodermal nuclei appeared. If I had had several embryos 

 presenting this same phenomenon, I should be inclined to think that 

 the entoderm was in process of lining the ectodermal vesicle and had 

 not yet completed its work; but, as I said in my description of this 

 embryo, the entodermal cells are so widely separated over the whole 

 vesicle that 1 do not feel justified in asserting that there is a space 

 really free from entoderm at this stage. Bonnet ('84), in his descrip- 

 tion of a sheep embryo of thirteen days, finds the entoderm in very 

 much the same condition ; he says : " Die Keimblase ist, wie audi die 

 Schnitte beweisen, durchweg doppelblattrig. Die Entoblastzellen bilden 

 aher in einiger Entfernung vom Schild keine continuirliche Lage, 

 sondern eine netzformig durchbrochene Membran anastomosirender 

 Zellen von 15-24 /xLange." Van Beneden ('80) finds that it is absent 

 at one pole of the vesicle, and the same condition has been observed by 

 both Heape ('83, Figs. 20-23, Plate XXIX.) and Keibel ('89, Fig. 46 a, 

 Taf. XXIV.). Hubrecht ('90) quotes Hensen (76, Fig. 18,Taf. VIII.) 

 as authority for the presence of the entoderm at this region, and the 

 figure would certainly seem to suggest this ; but we should at least not 



