210 
E. A. SCHAFER. 
Another proof, if one were needed, that the jelly is the pri- 
mary and the cells which wander into it the secondary part of 
the purely sustentacular mesoderm of these lowly organised 
animals is to be met with in the fact that in some — e. g. Hydra 
and the smaller Jelly-fish — the jelly is the only part of the layer 
present. The nutrition of the jelly is administered directly by 
the entoderm cells. 
We see then that in the lower forms the only function which 
is delegated to the intermediate layer is the mechanical one of 
support. But in all the higher animals the segregated mesoderm 
cells are deputed to perform other and more important functions 
as well, for, besides the connective and supporting structures of 
the body, the actively contractile tissues which are concerned in 
effecting the movements of the body are derived from them. In 
the Medusae or jelly-fishes this function is still performed by a 
tissue which is undoubtedly part of the ectoderm, although at 
first sight it seems to constitute a distinct layer of cross- striped 
fibres beneath the ectodermal epithelium. For if these fibres 
are carefully investigated, it will be found that they are many of 
them placed in the interior of large cells which project to the 
form. After a certain interval a more decided cupping again takes 
place, but this time it is the clear ciliated cells that are invaginated. This 
condition remains permanent. The cup-like sponge settles down and 
the orifice of the cup becomes closed; a jelly-like substance accumulates 
between the two layers of which the cup is formed, and cells pass into it 
from the outer layer ; one or more canals become bored through the 
jelly so as to effect a communication between the cavity of the sponge 
and the external medium ; and the sponge may be regarded as formed. 
Now it is generally assumed that the first cupping being temporary is a 
mere accident, and that the second represents the gastrula stage of other 
animals. If this is really the case there is of course no fault to find with 
the generally received interpretation which sets down the large cells of 
the ciliated chambers of a sponge as entoderm and all the rest of the 
sponge (viz. the flattened cells which cover the external surface and line 
the water-canals, and the general thickness of its jelly with its included 
branched cells), as ectoderm (and mesoderm). But it is difficult to avoid 
the thought that by ignoring the first cupping and setting it down as 
accidental or at least as unimportant on account of its transciency, an 
error may have been made and a divergent if not a retrograde process 
of development regarded as a normal and progressive "one. It is 
admitted that the microscopic appearance of the ciliated hemisphere of 
the sponge-blastosphere is in favour of its being looked upon as ectoderm 
rather than entoderm, and conversely with regard to the non-ciliated 
hemisphere. The origin of the branched cells in the jelly from the non- 
ciliated cells is a fact pointing in the same dii’ection. And although our 
knowledge of the physiology of the sponge is too imperfect to be of much 
service in deciding the question, nevertheless the vegetative mode of life 
which sponges exhibit would lead one to expect a proportionately in- 
creased entodermal and diminished ectodermal development. 
If the conjecture thus sketched out is a sound one it follows that 
sponges, as compared with other animals, are turned inside out, 
