1890.] The Mesoderm and the Calom. 889 
amnion, of which I shall give a fuller history elsewhere. The 
expansion does not take place by any means with the exact regu- 
larity indicated by Figs. 26 and 27, 
but, on the contrary, in birds, as 
shown by Zumstein, 46, the outline 
of the middle layer is always irreg- 
ular and more or less asymmetrical. 
Although there are not yet many 
observations available as to the out- 
line of the growing mesoderm, yet 
it is probable that the preceding 
description is essentially correct, not 
Fic. 27.—Diagram of the embryonic merely for birds but for all amniota. 
area of a chick. 4o, area opaca; AP, Tt is certainly so for the rabbit (Van 
area pellucida; 77, primitive streak ; à 
: 4 Beneden et Julin, 4). 
mes, mesoderm 
II. FORMATION OF THE CCELOM. 
Early in the course of development there appear in the meso- 
derm two large cavities, one on each side, which together constitute 
the cælom or embryonic body cavity. In the adult mammal the 
coelom is represented by the pericardial, pleural, and abdominal 
cavities ; the coelom also gives rise to the cavities of the muscular 
segments (protovertebrz), and probably also to certain tubular 
parts of the urogenital system. But, although its subsequent 
changes are complex, when it first appears the ccelom consists of 
a pair of fissures in the mesoderm. 
Only one precise account of the development of the caelomatic 
fissures is known to me, namely, that of Bonnet, 6, 202, for the 
sheep at about thirteen days. Around the embryo, at some dis- 
tance from the axis, there appear a series of irregular fissures of 
rounded or elongated form, which may in part open on the meso- 
dermic surface; gradually the fissures enlarge and fuse, at the 
same time becoming more closely bounded by the mesodermic 
cells; thus there arises a continuous cavity in the mesoderm, 
which is for a time crossed by cells and cell processes; mean- 
while the cells which are loosely put together form a compact 
layer of epithelium bounding the cavity, which we can now 
designate as the cao, or primitive body cavity. By similar 
