1887] > Metschnikoff on Germ-Layers. 345 
“We are justified in speaking of a form as a gastrula as soon 
as it can be made probable that the absence of both structures 
(blastosphere and gastric cavity) is of secondary origin. What we 
call a planula is, therefore, a gastrula formed by delamination.” 
These assertions are made without adducing any grounds for their 
probability, and without making it in the least degree conceivable 
how invagination can be abbreviated into delamination, or what 
coenogeny could effect for the origin of the latter. When one 
considers, moreover, that invagination is concentrated at one end 
_ of the embryo, and in the Medusz is confined to a relatively small 
area of the blastoderm, while primary delamination or multipolar 
invagination takes place at the most various points of the embryo, 
it is evident that a reduction of the two latter methods to the for- 
mer would meet with invincible difficulties. It is easy to under- 
stand how an invagination, originally confined to a small area, 
may gradually extend until, as is seen in various animals, it in- 
volves half the blastoderm. Further, one can see how a continu- 
ous layer of cells, destined to form the endoderm, may be changed 
into a cellular mass which is gradually enclosed by the growth of 
the ectoderm. But where the origin of the endoderm is an inter- 
rupted one,—that is, where the endoderm cells do not lie all to- 
gether, but alternate with ectoderm cells (compare the develop- 
ment of A‘ginopsis), or where the endoderm appears as the central 
segments of the blastoderm cells,—it is impossible to refer the 
„process to an abbreviated invagination. Multipolar immigration, 
to be sure, can be forcibly reduced to a number of invaginations, 
on which view each primitive gastric cavity would be represented 
by a single cell! It only needs to formulate such a hypothesis 
to demonstrate how utterly untenable it is; but, aside from this 
consideration, one would gain very little by accepting it, for pri- 
mary delamination would still remain totally unexplained. It is 
between unipolar immigration and invagination that a relation- 
ship can fairly be assumed to exist, as has been maintained by 
Claus and others. It is impossible for me, however, without a 
previous discussion of other questions, to decide which form must 
be regarded as the more primitive. 
Although its inability to explain the multipolar formation of 
endoderm is the weightiest objection to the Gastrza theory, it is 
by no means the only one. The theory was formulated at a 
time when the occurrence of intracellular digestion among the 
