258 F. M. BALFOUii. 
from the Protozoa, it is nevertheless worth while reviewing 
some of the processes by which this can be conceived to 
have occurred. 
On purely a priori grounds there is in my opinion more 
to be said for invagination than for any other view. 
On this view we may suppose that the colony of Pro- 
tozoa in the course of conversion into Metazoa had the 
form of a blastosphere ; and that at one pole of this a 
depression appeared. The cells lining this depression we 
may suppose to have been amoeboid, and to have carried 
on the work of digestion ; while the remaining cells were 
probably ciliated. The digestion may be supposed to have 
been at first carried on in the interior of the cells, as in the 
Protozoa ; but, as the depression became deeper (in order 
to increase the area of nutritive cells and to retain the food) 
a digestive secretion probably became poured out from the 
cells lining it, and the mode of digestion generally character- 
istic of the Metazoa was thereby inaugurated. It may be noted 
that an intra-cellular protozoon type of digestion persists in 
the Porifera, and appears also to occur in many Coelenterata, 
Turbellaria, &c., though in most of these cases both kinds 
of digestion probably occur simultaneously.^ 
Another hypothetical mode of passage which fits in with 
delamination has been put forward by Lankester, and is 
illustrated by fig. 8. He supposes that at the blasto- 
sphere stage the fluid in the centre of the colony acquired 
special digestive properties ; the inner ends of the cells 
had at this stage somewhat different properties to the 
outer, and the food was still incepted by the surface of 
the cells (fig. 8, 3). In a later stage of the process the 
inner portions of the cells became separated off as the 
hypoblast, while the food, though still ingested in the form 
of solid particles by the superficial cells, was carried through 
the protoplasm into the central digestive cavity. Later (fig. 
8, j), the point where the food entered became localised, and 
eventually a mouth became formed at this point. 
The main objection which can be raised against Lankester’s 
view is that it presupposes a type of delamination which does 
not occur in nature except in Geryonia. 
Metschnikoff has propounded a third view with reference 
to delamination. He starts as before with a ciliated blasto- 
sphere. He next supposes the cells from the walls of this 
to become budded off into the central cavity, as in Eucope 
* J. Parker, “ On the Histology of Hydra fusca’' ‘ Quart. Journ. Micr. 
Science and El. Metschnikoff, “ Ucb. die lutracellulare Verdauuug boi 
Coclenterateu,” ‘ Zoologischer Auzeiger/ No. 5G, vol. hi, 1880. 
