248 A Sketch of Comparative Embryology. [ April, 
casion to recur to this matter in speaking of the development 
of vertebrates. i j 
The embryology of sponges is important frontiè they do not 
have any gastrula. It will be described in our next article. - Ex- 
cept in the sponges, the small cells form the outside layer and are 
called the ectoderm, while the large cells form the inside layer, or 
entoderm. In England the attempt has been made to substitute 
epiblast for ectoderm, and hypoblast for entoderm, but the change 
seems to me useless and confusing. In face of the present ten- 
dency to substitute new and difficult for old and simple names 
every protest is desirable. Compounding English polysyllables 
from Latin and Greek confers, in most cases, no benefit to science. 
The coining of such terms ought to be restricted in its applica- 
tion to things which have no accepted name and for which no 
straightforward English term can be found. 
The next Pi after the formation of the ectoderm and ento- 
derm does not occur 
among all animals, but 
> only in those above the 
/  Ceelenterates. I refer to 
the development of a dis- 
tinct middle layer of cells, 
the mesoderm, situated as 
shown in Fig. 15, be- 
tween the two primitive 
layers. Of the origin 
and characteristics of the 
mesoderm I shall treat in 
the next article. 
Fic. 15.—Section of a al sine of oho cept A great many embryos 
lividus, after Selenka; — live in the water, and have 
the power of locomotion long before they have any muscles. 
For this purpose the ectoderm in these forms is provided with 
cilia or vibratile hairs, which may be longer (Fig. 15) or shorter. 
In most free embryos, moving by cilia, we find distinct bands, — 
along which the cilia are more developed and powerful ; as the 
ciliated bands are often pigmented, while the rest of the embryo 
is transparent or light-colored, SA are very conspicuous. We 
shall have to recur to them. 
The authorities for the general views advanced above are the _ 
