424 Metschuikoff on Gerin-Layers. [ May 
From our point of view the endoderm must be regarded as an 
aggregate of cells, which were originally derived from the blas- 
tosphere by immigration or transverse division, and which then 
became associated together to form a mass of amceboid ele- 
ments. The gastric cavity, as well as the mouth, must be re- 
garded as later acquirements, whose appearance, however, in the 
ontogeny of certain forms has been so accelerated as to lead to 
the direct production of a gastrula. 
Balfour (29), who has declared against the parenchymella 
theory, admits “ that it fits in very well with the ontogeny of the 
lower Hydrozoa.” Now that our knowledge of the facts is 
much greater than it was when this quotation was written, the 
harmony is still more marked. Balfour says in the same place 
(vol. ii. p. 285) that the passage from the protozoan to the meta- 
zoan state postulated by this theory strikes him as “ very im- 
probable in itself.” But I cannot answer this criticism, since the 
assertion is made without any attempt to Support it by argu- 
ment. Much more precise are Bitschli’s objections, which, 
however, concern the physiological side of the question exclu- 
sively. After a short account of my views, he remarks as fol- 
lows (l. c., p. 418): “It seems to me that the endoderm cells, 
whose special business it is to take in food, would lose rather 
than gain by migrating into the interior of the colony. ‚Without 
the simultaneous formation of a mouth, for which neither this 
nor Lankester’s hypothesis can offer any reason, the immigration 
of the endoderm cells would be a decided disadvantage, since 
they then, so to speak, lock themselves in.” When I first de- 
scribed my theory of the formation of endoderm by immigra- 
tion (8), I referred to Volvox, in which the reproductive individ- 
uals leave the surface of the colony for the central cavity. At 
the present time I can also refer to the immigration of individu- 
als that takes place in Protospongia, and which is certainly not 
without advantage to the colony in general. There are, more- 
