NO: 2.| DEVELOPMENT OF THE HUMAN COELOM. 409 
specimen, but which unfortunately is an abnormal one. My 
interpretation of these three specimens (Nos. XIII, XIV, and 
XX) is that the fibrous degeneration overtook the embryonic 
vesicle after it had reached the stage of Graf Spee’s embryo 
v. H., my Fig. 14. The remaining three embryos (Nos. XXI, 
XX XVII, and LVIII) are of the vesicular form, and I believe 
them to be especially valuable for the proper interpretation of 
the early stages of development of the human coelom. 
Nos. XXI and LVIII came to me as perfect specimens, 
both having been hardened unopened, the first in strong 
formalin and the second in strong alcohol. No. XXI was still 
enclosed in its decidua, 
and appeared to be a 
normal specimen until it 
had been cut into serial 
sections. The embry- 
onic vesicle proved to 
be very large, and was 
composed throughout of 
two layers, an inner one 

giving all the appearance 
of the entoderm and an fig. 7. — Diagram of a Pathological Ovum which rep- 
outer giving all the ap- 
pearance of the mesoderm of the umbilical vesicle of young 
embryos. The mesodermal layer contained within it islands 
of blood cells, as are also present in normal specimens. The 
whole vesicle was connected to the chorion with a mass of 
mesodermal cells somewhat as shown in the diagrammatic 
Fig. 7. The chorion and decidua appeared to be normal. 
No. LVIII showed considerable change in the mesoderm of 
the vesicle and chorion, giving somewhat the appearance of 
fibroid degeneration rich in cells. The chorion was attached 
to the vesicle by a strong pedicle, as shown in Fig. 7. The 
vesicle itself was composed of two layers, an inner and contin- 
uous one composed of one layer of cells, and an outer and 
thickened layer appearing like the mesoderm of the chorion. 
There were no indications of blood islands. In addition to 
these two layers there was a third layer fairly well marked near 
resents an Early Hypothetical Stage. 
