No. 2.] DEVELOPMENT OF MANICINA AREOLATA. 199 
deeper hue, and is therefore well defined. On comparing this 
figure with Figs. 6 and 10, there seems to be no doubt that the 
mass in question really represents the ectoderm base of the 
cesophagus. 
The central portion of the larval endoderm remains as a food- 
yolk. In Fig. 5 it still exists as a continuous structure, but as 
more and more of the protoplasm is drawn into the layer of per- 
manent endoderm, the vesicles lose their connection with each 
other. By the time the layers are definitely established, Figs. 
6 and 8, the yolk is a loose mass of vesicles, the shells of which 
have begun to disintegrate. The shell is extremely dense, and 
in this and subsequent stages seems to be fatty, as it stains very 
dark with osmic. Remnants of the yolk are found in stages as 
late as Fig. I9. 
Though the formation of the supporting lamella and the dif- 
ferentiation of the permanent endoderm very often take place 
at about the same time, this is not always the case. In many 
larve the supporting lamella is entirely formed, and the cesoph- 
agus has opened centrally, while the endoderm is still solid, 
Figs. 10 and 12. 
The “yellow cells,” which later are found in such abundance, 
appear for the first time in the planula, Fig. 4. I have not 
seen them actually entering the planula, but in this and slightly 
older stages, Fig. 5, a few occur in the surface ectoderm, 
whereas in older larve and in the adult they are confined to 
the inner layer of the body. 
COMPARISONS. 
a. Germinal Layers. 
Among the many ways in which the germinal layers are 
formed within the Ccelenterates, Metschnikoff (2) has decided 
that one represents the manner in which the earliest metazoa 
were formed. In this so-called “mixed delamination” a solid 
endoderm is built up both by delamination and by the migration 
of superficial cells into the interior. Polyxenta (Metschnikoff) 
and, better, Aurelia (Gotte) are good examples of this process. 
Accepting this type as ancestral, JZanzcina has diverged along 
the same path as certain Trachomedusze (Geryonia, Liriope) ; 
