762 
The All 
Naturalist. 
term diaderm ; it is characterized by consisting of two plates of 
cells, differing in character, joined at their edges (ectenal line), 
and surrounding a central segmentation cavity : the two plates or 
laminae are the two primitive 
germ-layers, the ectoderm and 
entoderm. The earliest form of 
the diaderm is that known as the 
blastida, as Haeckel has fel- 
icitously named the first larval 
form of the lower animals. In 
the blastula we have a simple 
epithelial vesicle, the cavity of 
^^^'- '7- which is the large segmentation 
^ST'^ixTr^lXt"'"'^'^^''"''' cavity (Fig. 1 6); the^ epithelial 
layer is one cell thick and divided into two regions, the one com- 
posed of smaller cells is the ectoderm, Ec, and the other, of larger 
cells, is the entoderm, en. This stage occurs with sundry modi- 
fications in a great many invertebrates. These modifications are 
due principally to the increase in the size of the entodermic cells, 
which, as already pointed out, results from 
the increase of the yolk matter in the ovum. 
Thus in many mollusks the entodermic 
cells are very large, and at first, few in num- 
ber, (Fig. 17). By a still further modifica- 
tion the cellular yolk is replaced by a mass 
rich in deutoplasm, but not divided by cells, 
while at the same time the segmentation 
cavity is reduced by the invasion of the yolk 
mass. This is well exemplified in the ova 
of many arthropods, (Fig. i8). We have 
in this case the blastula type still evident, 
although the entoderm has at this stage no o.,. .. VJ. ' ; ,,:. • ' ,,tu . 
trace of its epithelial structure. In verte- iemr' Iv'; viteUus oi vulk 
brates we have the additional modification ^^P^-^^^^t'^g the entoderm. 
that cells are several layers deep in the ectoderm, and primitively 
m the entoderm also.-compare the sectio 
(Fig. 4). In 
I of the frog's > 
, the entoderm 
