No. 2.] DEVELOPMENT OF MANICINA AREOLATA. 197 
cells, the ectoderm. The inner ends of these cells are not dis- 
tinctly marked off from the solid endoderm. The endoderm, 
when the living larva is compressed or when pretty thick 
sections are examined, appears made up of a quantity of vesicles, 
which look something like fat cells. When thin sections are 
examined (Fig. 4), it is found there exists a continuous proto- 
plasmic matrix binding the vesicles together, which are now 
seen to be identical with the vacuoles present in the endoderm 
segments of the blastosphere cells. There is, however, a differ- 
ence in their appearance, which is best shown in Figs. 9 and Io. 
In the blastosphere cell, the protoplasm in which lie the vacu- 
oles is pretty uniform ; but in the endoderm of the planula, the 
protoplasm immediately round each vacuole is denser than the 
intervening stroma, and forms a more or less differentiated shell 
for the vacuole. The shell stains much more deeply than the 
stroma. What the contents of the vesicle is, I cannot say. 
Carmine and hematoxylin do not affect it, and in these young 
stages I did not try osmic acid. 
The endoderm contains, besides the vacuoles just mentioned, 
a number of “yellow cells” and scattered nuclei. No cell 
boundaries can be made out, and there is every indication that 
the layer is a plasmodium. A stage between Figs. 2 and 4 
would go far towards elucidating the intricate structure of the 
solid endoderm, but I did not succeed in obtaining such. 
While the embryo is still solid, the cesophageal invagination 
makes its appearance, Fig. 4. The larva is very narrow except 
at its base, where it is slightly dilated. At this stage it swims 
feebly about, the cilia having commenced to develop. 
The formation of the permanent layers is illustrated in Fig. 5. 
In the region @ the layers are in the condition characteristic of 
the earlier larva. But in the neighborhood of ¢ the ectoderm 
cells have become much more clearly marked off from the 
endoderm. Their inner ends forma ragged line which gradu- 
ally becomes more uniform, until the ectoderm is bounded 
internally by a smooth limiting surface, as at a. Though the 
bounding surface is smooth in this region, the supporting 
lamella has not yet formed. A little later it appears as a 
delicate membrane between the two layers (from @ towards the 
mouth). While it is still very thin it resembles an ordinary 
cuticle. It appears between the layers, and is not formed by 
