No. 2.] DEVELOPMENT OF THE HUMAN COELOM. 403 
deal ; they only show us that the monkey’s development is like 
that of man. In monkeys we have also the precocious chorion 
and the early amnion and the large coelom between the 
umbilical vesicle and the chorion. The marked difference is 
that the amnion is attached to the chorion along its dorsal side, 
while in the human embryo this is only the case along the pos- 
terior end of the amnion. The attachment of the amnion 
along the chorion suggests that the embryonic plate separated 
from the exterior of the ovum along this point, as Selenka 
thinks he observed in a very young ovum only 1.5 mm. in 
diameter. Unfortunately, the most valuable specimen was 
injured in its preparation,! and Selenka did not trust himself 
to give any illustrations of it. 
With the amnion attached at its dorsal end to the chorion, 
we understand why the entodermal end of the allantois must 
grow around an angle to reach the chorion (Selenka, Plate 
XXXVII, Fig. 5). Somewhat the same arrangement has been 
described by Graf Spee? in his embryo Gle., but the curve is 
by no means as marked, indicating that the attachment of the 
embryo to the chorion is along its posterior end, as shown 
by His? in his well-known diagram of the formation of the 
amnion. 
Regarding the very early stages of monkeys and man it is 
better that we make comparisons with animals most nearly 
related to them, and now we have careful studies of the very 
early stages of Chiroptera at our disposal. I believe that 
Selenka’s* study of the development of Pteropus edulis gives 
us the key for the comparison of the formation of the blasto- 
dermic membranes in mammals. Recent investigations by 
Duval® on different families of Chiroptera appear to confirm 
the work of Selenka on Pteropus. 
In order to illustrate these points more clearly I have made 
diagrams of three of Selenka’s figures of Pteropus. Fig. 1 is 
from an ovum covered completely with two layers of cells, 
1 Selenka: Studien, 1891, p. 201. 
2 Graf Spee : His’s Archiv, 1896, Taf. I, Fig. 1. 
8 His: Anat. mensch. Embryonen, Theil I, p. 171. 
£ Selenka: Studien, 1892, p. 2009. 
5 Duval: Jour. de l’Anatomie et de la Physiologie, 1895. 
