346 Metschnikoff on Germ-Layers. [April 
lower Metazoa was not known, when in fact digestion in all cases 
(Metazoa) was believed to be enzymatic: naturally it is now un- 
able to answer the questions suggested by our advanced physio- 
logical knowledge. The Gastræa theory would compel us to 
believe that a deep gap intervened between the one-layered blas- 
tosphere and the double-walled gastrula with its digestive cavity. 
This very awkward gap is, however, easily filled as soon as we 
abandon the Gastrzea theory and seek to explain the origin. of 
the endoderm in another manner. I need not dwell here upon 
the difficulties encountered by supposing all the known gastrula 
forms to be homologous. Such matters are not directly con- 
nected with our discussion of the primitive condition of the en- 
oderm, and besides I shall have something to say on this point 
in another place. 
The Planula theory of Ray Lankester (27) is based on the 
development of the Geryonidz, and considers the method of 
forming the endoderm here employed, by constricting off the 
inner ends of the blastosphere cells, as the primitive type. Lan- 
kester endeavors to derive invagination from a primary delamina- 
tion, such as occurs in Geryonia. But even were we satisfied 
with this derivation, there would still remain unexplained the 
cases where there is no actual delamination, but where the endo- 
derm is formed of cells which migrate from various points of 
the surface into the interior of the embryo. The significance of 
this latter origin receives additional strength from unipolar im- 
migration, the endoderm cells in both cases being blastoderm 
cells, which have arisen by longitudinal division from previous 
blastoderm cells. Moreover, the same objection must be raised 
to the Planula theory as to the Gastræa theory, namely, it rests 
on the assumption that digestion in the lower animals is enzy- 
matic, and herein contradicts our actual physiological knowledge. 
Lankester believes that the formation of a cavity into which a 
> secretion was poured preceded the formation of the 
endoderm. In other words, the inner segments of the blasto- 
derm cells functioned as digestive elements while the polyplast 
(blastula) was still one-layered. All these assumptions become 
quite inadmissible when once we learn that intracellular diges- 
tion persists in many of the lower Metazoa, and is even found in 
some Molluscs (Phylliroé). 
-W ee E T A a O 
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