1887] _ Metschnikoff on Germ-Layers. 419 
METSCHNIKOFF ON GERM-LAYERS.' 
TRANSLATED BY H. V. ‘WILSON. 
(Concluded from page 350.) 
F we consider all that has been said on the theories in question, 
we cannot but see that they fail to establish the connection 
between the various embryological phenomena,—to combine them 
under one point of view, so to speak,—and that, moreover, they 
display a lack of physiological explanation. Some other theory 
must, then, be invented. In my studies on the Sponges (8) I very 
cautiously made a few remarks which, as I thought, agreed with 
what we knew of the way in which the endoderm was formed 
among the lower Metazoa, and which could be brought into har- 
mony with the phenomena of intracellular digestion. I stated 
my belief that the endoderm did not appear in the beginning as 
a sac-like stomach with a terminal opening, such as one finds in 
the gastrula, but that behind these structures lay a long historic 
process, recorded in the formation of a solid parenchyma, in which 
digestion is intracellular. This parenchyma did not appear all 
at once, but was gradually formed from superficial blastoderm 
cells that migrated into the central cavity. There finally arose 
a two-layered parenchymella, which, by abbreviation of the em- 
bryonic process, along with the advancing differentiation of the 
digestive apparatus, became changed into a gastrula. At that 
time (1879) it was impossible for me to refer to any highly- 
developed Flagellate with animal nutrition. I therefore tried to 
find some foundation for my views in the development of Volvox, 
and in this connection made the following remark: “ In my opin- 
ion it is time to begin looking for some low organisms in which 
the nutritive cells, perhaps after having taken in food, leave their 
usual position at the surface of the ‘colony,’ and come to lie 
within the central cavity” (p. 382). Shortly afterwards (July, 
1880) Saville Kent discovered a most interesting form of Flagel- 
late colony, which he introduced to science as Protospongia 
hackelii (20). The individuals of a colony are at first regularly 
arranged at the surface. Some of these assume a pronounced 
amceboid shape, and migrate into the interior of the mass of 
The numbers in this article refer to the bibliographical list appended, _ 
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