No. 2.] ANURIDA MARITIMA. 269 
matin in cells with a strongly assimilative function. In the 
second case, when these cells remain permanently associated 
in one or more clusters, no further change occurs until late in 
embryonic life. By the subsequent development of the embryo 
and its changes in form during flexure, the relative positions of 
the groups are somewhat changed. One large mass may, how- 
ever, readily be recognized in the region of the proctodaeum, 
not far from the ectodermal layer. 
After the embryo has reached a stage corresponding to Fig. 
45, or perhaps later, the mass may be seen to be scattering, and 
certain changes occur in the yolk. Around some cells are large 
spherical masses of yolk particles contained in extremely vesic- 
ular protoplasm, in which there is a small central nucleus (Fig. 
58, ez.). These are particularly numerous in the regions of the 
stomodaeum and proctodaeum, and may be clearly seen later 
to assume a regular arrangement in the yolk, forming two 
broken lines reaching through the body. It is now clear what 
these mysterious cells are: they are the entoderm, and are 
taking up definite positions to form the mesenteron of the 
young animal. In a newly hatched specimen an interesting 
relation is shown; Fig. 65 represents part of a frontal section 
through the body of such an animal. The mesenteron is seen 
to be composed of large irregular-shaped cells with extremely 
vesicular protoplasm; the nuclei are irregular in size and stain 
faintly. At the inner or free edges of the cells masses of yolk 
are visible, and certain of the cells also contain similar particles. 
Whether these particles are passing out of the entoderm cells 
to the enteric cavity or are being ingulfed by them is not clear, 
but in either case it is evident that the mesenteron when fully 
formed contains very little food yolk, a condition contrary to 
the general rule. The entodermic cells are resting on an 
extremely thin membrane, but there is as yet no sign of muscu- 
lar walls or other differentiation. The cells are themselves still 
irregular and almost amoeboid in form (Fig. 65, ¢7.). 
This is, then, the history of the entoderm in Anurida; it 
originates without doubt during cleavage, and takes up its posi- 
tion in the middle of the morula by a process which it is possible 
to call invagination. When the mesoderm, which also lies within 
