1887] Metschnikoff on Germ-Layers. 339 
of the above-mentioned primitive organs, should only be given 
the rank of theories when they are in naro with our actual 
knowledge. 
The transitional stages between the Protozoa and Metazoa do 
not appear to exist at the present day. Endeavors have been made, 
however, to fill this gap in our knowledge by means of hypothetical 
organisms. There are two ways possible for such a transition to 
take place, —either by a differentiation of protoplasm around the 
separate nuclei of a multinucleate protozoan, or by the union of 
the several individuals of a protozoan colony into a many-celled 
individual. We will discuss the former method of transition first, 
and then take up the latter. A close relationship was some time 
ago supposed to exist between the ciliate Protozoa on the one 
hand, and the Turbellarians on the other, especially the larve of 
. the latter. On the supposed kinship between the two groups 
there have been built up hypotheses relating to the descent of 
the Metazoa. Such hypotheses have as their kernel the transi- 
tion of multinucleate Protozoa into Metazoa, and have been 
adopted by several investigators, among whom we must men- 
tion Jehring (19) and Saville Kent (20). From the stand-point 
here taken, the mouth and anus of the Infusoria are homologous 
with the like organs in the Metazoa. Indeed, Jehring believes 
that the water vascular system of the latter has been derived 
from the contractile vesicle of the Protozoa. Looking over the 
whole field of embryology, we find the formation of the blasto- 
derm in Insecta to be the process most in accord with this hy- 
pothesis. It is, in fact, on the first stages in the development of 
the Aphides that Kent mainly rests his belief. Considerations 
of this kind clearly show that such a hypothesis cannot be main- 
tained. While ignoring all the embryological facts of the lowest 
Metazoa, the theory harmonizes with the formation of the blasto- 
derm in the Insecta; that is, in a group which has suffered in. 
every respect great secondary changes. But even in this group 
there are forms that contradict the hypothesis, as, for example, 
the Poduridz, insects which in other respects occupy the lowest 
position in the class, and agree in the segmentation of the egg 
with Myriapods. When the facts are these, no value can be as- 
cribed to the homologies. drawn between the mouth, anus, and 
water vascular system of the Infusoria and Metazoa. 
On the other hand, the —— which supposes that colo- 
VOL. XXI.—NO. 4. 
