406 MALL. [Vou. XII. 
and 12 (Plate XX XV) and figure 6 (Plate XX XVII) on monkeys, 
will decide this question more definitely than all the many 
discussions on the human chorion put together have done. 
Having now selected from Selenka diagrams and descriptions 
of the development of the germ layers of Pteropus, it is easier 
for me to give a plausible explanation of the beginning of the 
coelom in the human embryo. If the diagram I have given 
in Fig. 3 is compared with Selenka’s figures 5 and 11 (Plate 
XX XV) and figure 5 (Plate XX XVII) of the monkey, as well 
as with the sections of young human ova published by Graf 
Spee! and by myself,? one is struck with the great similarity 
of the two groups of figures. 
Fig. 14, given further on, is a diagrammatic outline of a 
longitudinal section of a young human embryo published re- 
cently by Graf Spee. It is the one marked v. H. in the table 
of young human ova given in the beginning of this paper. 
When, now, this section is compared with the transverse sec- 
tion of Pteropus, in Fig. 3, the only marked difference is that 
the umbilical vesicle in Pteropus has retracted, in order to make 
the arrangement of the membranes as given for the human 
embryo in Fig. 14. 
In order to make the connection complete, I give hypotheti- 
cal stages in Figs. 4, 5, and 6. Fig. 4 represents the human 
ovum in the two-layer stage. The outer layer, or Rauber’s 
layer, is complete as in the rodents and in Pteropus. The 
inner layer, or entoderm, is also complete. Between the two is 
the embryonic shield, or ectoderm of the future embryo. The 
next figure, 5, shows the beginning of the mesoderm develop- 
ing towards the tail end of the embryo, as this is the position 
of the primitive streak, and as the head fold of the amnion 
in many embryos is often only invested with ectoderm and 
entoderm. A stage later, Fig. 6, finds the mesoderm envelop- 
ing the umbilical vesicle completely, and also partly lining the 
outer layer, R, of the ovum. The cavity between the two is the 
coelom. At the tail end of the embryonic disc the mesoderm 
1 Graf Spee: His’s Archiv, 1889 and 1896. 
2 Mall: A Human Embryo of the Second Week, Anatom. Anz., Bd. 8, and 
Early Human Embryos and the Mode of their Preservation, Johns Hopkins Hos- 
pital Bulletin, 1893. 
