DEVELOPMENT OF THE HUMAN COELOM. 
FRANKLIN P. MALL. 
Four years ago I wrote a general article on the coelom for 
Wood's Handbook of Medical Science, in which was emphasized 
the separation of the body cavity from the extraembryonic 
coelom. Since then I have had opportunity to extend my 
observation to the human embryo, and therefore make this 
communication. 
Unfortunately, there are no data regarding the beginning of 
the coelom in the human embryo, and in all probability none 
will ever be found. The smallest human ovum ever seen is 
that described by Reichert.1 It was obtained from a woman 
who had committed suicide, on account of pregnancy, forty-one 
days after the beginning of the last menstrual period. It was 
therefore presumably about thirteen days old. This ovum, 
which is pictured in every text-book, was 5.5 X 3.3 mm. in 
diameter, was surrounded by a zone of villi leaving two poles 
bare, and contained in its interior a mass of cells measuring 
1.5 X 1.75 mm. All the space between this inner mass and 
the chorion is the coelom, and regarding its origin, we can no 
more than speculate. 
During the last few years three other human ova, slightly 
larger than Reichert’s, have been cut into sections, thus per- 
mitting a more careful study of their contents.2- The dimensions 
and approximate ages of these embryos are given in the 
table on the following page. 
It is noticeable that in the three embryos just mentioned, as 
well as in the remaining four of the table, the size of the whole 
egg does not correspond with the size of the embryo, nor with 
its age. I do not think that this great variation in the size of 
the chorionic vesicle is altogether due to the method of harden- 
1 Reichert: Abhandl. d. kgl. Akad. d. Wiss., Berlin, 1873. 
2 Von Spee: His’s Archiv, 1889; Mall: Anatom. Anzeiger, 1893; Johns Hop- 
kins Hospital Bulletin, 1893; and von Spee: His’s Archiv, 1896. 
