402 MALL. [Vou. XII. 
like a paradox, but, since the comparative methods of study 
have been introduced, inversion only means that the amnion 
is completed before the medullary groove begins to form. 
This alteration of the development of the amnion and the 
medullary groove makes the body of the embryo develop on a 
concave surface instead of on a convex one, thus apparently 
making the embryo inverted, as is the case in the guinea pig. 
Closely associated with inversion of the blastodermic mem- 
brane is the formation of an additional layer of cells, discovered 
by Rauber,! the importance of which has been emphasized by 
Selenka and others. Rauber’s layer is so marked in the rabbit 
that it was at first believed to be the true ectoderm. The fate of 
Rauber’s layer has not been sufficiently studied to interpret it 
completely, and our ideas regarding it will not improbably 
require some revision. In many rodents Rauber’s layer becomes 
markedly thickened on one side of the ovum, forming a support, 
or 7rager, for the ovum. The relation of Rauber’s layer to 
the Trager is shown beautifully by Selenka? on Plate XVI of 
his monograph. 
The question which interests us here is whether the inver- 
sion of the blastodermic membrane as well as the discovery of 
Rauber’s layer aids us in advancing a theory of the develop- 
ment of the germ layers of the human embryo, and thus in 
turn to explain the large coelom as found in all of the earliest 
human ova. I realize fully that any such effort will not be 
final, yet I believe that it will aid us to understand better the 
relation of the membranes as found in the human ovum. 
In looking over the illustrations of the development of ani- 
mals closely related to man, one is struck with the similarity of 
the arrangement of the membranes to those described for the 
human ovum by Graf Spee. One must compare only plates 
XXXV-XXXVIII of Selenka’s® paper with the two plates 
accompanying Graf Spee’s* to be convinced that the early 
development of monkeys is almost identical with that of man. 
Yet Selenka’s researches on monkeys do not help usa great 
1 Rauber: Sitzungsber. d. Naturforcher Gesellsch., Leipzig, 1875. 
2 Selenka: Studien iiber Entwickl. d. Thiere, Heft 3, 1884. 
3 Selenka: Studien, efc., Heft 5, Erste u. Zweite Halfte, 1891, 1892. 
4 His’s Archiv, 1889 and 1896. 
