594 J. T. PATTERSON 
under side of the ectodermal vesicle, at a point where the embry- 
onic and amniotic portions of the vesicle meet on the left side. All 
such straggling entodermal cells show signs of decadence, and 
without doubt play no réle in the development of the mesoderm. 
The first evidence of mesoderm formation is also found in this 
same vesicle, and is indicated by the beginning of a process of pro- 
liferation involving the ectodermal cells which lie at the point or 
angle where the entoderm parts from the ectodermal vesicle. In 
figure 55 one of the proliferated cells, which has just been set free, 
is seen on the right, slightly removed from the angle. Only a very 
few such cells are found about the edge of the vesicle, but the 
presence of mitotic figures in this general region of the ectoderm 
indicates the approach of a rather profuse proliferation of meso- 
dermal cells. 
In another vesicle, somewhat larger than the preceding, the 
formation of the mesoderm has made rapid progress. The cells 
are proliferated in clusters, and soon develop into small vesicular 
structures (figs. 56,57). In this particular specimen there are about 
eight small mesodermal vesicles, but it was not possible to deter- 
mine whether their origin was confined to one or more localized 
regions of the ectodermal vesicle, or centers of active prolifera- 
tion. The preparation shows every stage in the formation of ves- 
icles. In most instances the cluster of cells is set free from the 
ectoderm before a cavity appears within their midst; in others, the 
cavity arises while the cells still retain a connection with the ecto- 
derm. In all cases the smaller mesodermal vesicles gradually fuse 
together to form larger and larger cavities, until finally the entire 
space lying below the ectoderm is lined with a mesodermal layer. 
While I was unable to determine any definite localized regions 
of mesoderm proliferation in this specimen, in all the older stages 
the mesodermal vesicles fuse in such a way as to produce two main 
vesicles. For example, specimen No. 234 has two well differen- 
tiated mesodermal vesicles, which are unequal in size (fig. 19). 
The larger one on the left-hand side is composed of a typical meso- 
dermal layer. In this region it is free from any connection with 
the ectodermal vesicle, but in other sections it not only has such 
connections, but is also united to other smaller mesodermal vesi- 
