430 Metschnikoff on Germ-Layers. [ May 
either solved or escaped. In this connection I refer to my 
“Studies in Comparative Embryology” (10), where I have dis- 
cussed this side of the question. Blochmann (37) and Sedgwick 
(38) have recently endeavored to rescue the gastræa theory by 
once more propounding the view that the mouth and anus have 
both been formed from a slit-like blastopore. The evidence on 
which this view is based consists of Balfour’s study of Peripatus, 
and of observations on Aplysia and other Gasteropods. The 
authors believe, however, that a slit-like blastopore which gives 
rise to both mouth and anus, may be assumed to occur in the 
Metazoa generally. But the gastraa theory is not thus freed of 
its main burden, for, if we accept this assumption, the radial 
gastrule of the Echinoderms, Pilidium, and Polygordius must 
be looked on as larval forms secondarily modified to a great 
degree, while the embryos of the Gasteropods, Peripatus, Insects, 
and Worms with a slit-like blastopore would represent the con- 
dition of the primitive gastrula. In like manner the regular 
blastospheres of the former animals would have to be regarded 
as the modified descendants of the amphiblastulz, rich in yelk, 
of the latter. 
The genealogy of the anus, which is not satisfactorily eluci- 
dated by the theory just discussed, is to be traced in a series of 
stages such as we have assumed to occur in the development of 
the mouth. In the lower Metazoa we observe two (Ctenophora) 
or more openings for the exit of the excreta, just as in the 
Sponges there are numerous openings for the entrance of food. 
In some of the Medusz belonging to the family Lafceide 
(4Equorea, Tima) the numerous excretory openings of the gas- 
tro-vasculay system are seated on special papilla; some of the 
Polyclade possess similar excretory openings on various parts 
of the body. In Cycloporus (39) Lang observed the extrusion 
of some drops of fluid containing differently-colored concretions 
through such external openings of the digestive apparatus. This 
= Observation is all the more significant because the Polyclade 
possess, besides these openings, a special excretory system. 
While one portion of the phagocytoblast developed into the 
endoderm, in which the originally amceboid cells gradually as- 
_ sumed an epithelial character, another portion of the same prim- 
itive organ gave rise to the mesoderm. The latter originally 
appeared in the shape of solitary migratory cells, which contin- 
