213 
E. A. SCHAFER. 
set of cells from the entoderm and ectoderm, destined to sub- 
serve the functions of support and motion, and termed meso- 
derm — has in reality in all probability been produced by the 
coincident occurrence of at least two distinct segregations. Per- 
haps one of these, the muscular, is precocious, and has blended 
with the other, the sustentacular, which should have preceded it. 
If this is the case each of the two segregations should be re- 
garded as constituting a distinct stage in development. But they 
are so completely blended in their origin in the higher animals 
that it is impossible to differentiate them. 
There is no reason to suppose that the two layers into which 
the mesoderm subsequently splits, in animals in which a body- 
cavity, or coelom becomes formed, have anything to do with this 
supposed primary double segregation of the layer. Por the 
segregated cells are entirely intermixed as the mesoderm spreads ; 
although they may afterwards become again partially separated in 
groups (for the constitution of muscles, cartilages, &c.), according 
to the function for which they are destined. The formation of 
the coelom is a distinct forward stage in development, and is the 
first step in the direction of the formation of a circulatory 
system. The cells which bound both the coelom and its offsets 
are in most animals segregated from the general mesoderm, and 
belong to the set of sustentacular cells, but in the Holothurian, 
according to the description given by Selenka (and also in 
Sagitta and Amphioxus as shown by Kowalewsky), they are 
derived directly from the entoderm of the alimentary cavity, and 
already before their severance from this, enclose a commencing 
coelom. There is not sufficient ground for regarding these lining 
cells of the coelom and vascular system as constituting a special 
segregation. In vertebrates they are certainly of the same 
nature as those of the sustentacular part of the mesoderm (the 
connective tissue), and the same is probably the case in the other 
animals where they are found. 
Another well-defined forward stage in development is the 
occurrence of a special segregation of ectodermic cells for the 
performance of the nervous function. These differ from those 
which are destined for the muscular function in that they are 
never blended in their origin with the mesoderm, and indeed do 
not in any animal lose their primitive connection with the 
ectoderm until development is comparatively advanced, if even 
they do so then. 
As in the case of the muscular segregation the first indication of 
a separation of some of the ectodermic cells for the performance 
of nervous and special sensory functions is met with in the 
Coelenterata. This takes the form of a prolongation of the 
attached ends of some of the ectoderm cells into branched 
