EMBEYOLOGY OF STOMOTOCA APICATA 
341 
FORMATION OF THE ENTODERM 
In Stomotoca the formation of the entoderm takes place by 
unipolar ingression, or the ^'hypotrope" method. The latter 
term was used by Metschnikoff in contradistinction to multi- 
polar migration. In the multipolar formation of the entoderm he 
distinguished four different modes, namely : 1 . A primary delam- 
ination which takes place by a transverse division of the blasto- 
derm cells, and occurs in the Geryonidae and Eudendrium. 2. 
A multipolar ingression which takes place on all sides (Aeginop- 
sis). 3. A secondary delamination which occurs where a morula 
structure exists, as in Aglaura, Rhopalonema and in most of the 
hydroid polyps. 4. A mixed delamination in which the ento- 
dermal cells originate in part through transverse division or in- 
gression; and also through subsequent differentiation as a second- 
ary delamination. This last mode of the formation of the ento- 
derm, according to Metschnikoff, occurs in Polyxenia; and is the 
transitional method between multipolar migration andepibole. 
In the unipolar ingression, or ''hypotrope'^ process, the formation 
of the ectoderm is confined to a comparatively small area at the 
posterior end of the blastula. This is the method that is fol- 
lowed in the species under consideration. 
About the time the blastula becomes ciliated and begins to 
swim, usually eight to ten hours after fertihzation, the cells at the 
posterior end of the larva become somewhat taller than those in 
the other regions, and from these cells, relatively few in number, 
the entoderm arises. The formation of the entoderm in Stomo- 
toca is, in a general way, similar to that described and figured by 
Metschnikoff in his ''Embryologische Studien an Medusen" for 
Clytia flavidula, Clytia viridicans and Octorchis Gegenbauri. 
The entoderm cells are given off from the lower end of the blastula 
and are pushed into the blastocoele. At first a single cell may be 
budded off. Gradually more cells are given off, and those first 
set free divide; so that by the continuation of this process for an 
indefinite time, the blastocoele becomes filled solidly from the 
posterior to the anterior end. Fig. 18, 19 and 20 are from sections 
of blastulae in which the formation of the entoderm is in different 
