28 
structure with analogies to the organ of the same name in Insecta, 
the Pilidium development, &c., as Kennet had previously insisted. 
It would appear to me to be a membrane conditioned by the way in 
which the adult is formed upon the larva. 
Another important larval structure is the yolk-sac placenta 
of Mustelus laevis and of many Mammals!). In the latter the 
importance of this organ during a long period of foetal life has been 
proved by HUBRECHT and RoBINson. 
The yolk-sac placenta may be explained as due to the fixation 
of a parasitic larva, indeed, in Mammals the larva has become a fixed 
internal parasite in the uterus, and its mode of life, like that of all 
internal parasites, leads to great degeneration. 
In this connection it may be insisted that it would be contrary 
to all that we know concerning the effects of the parasitic mode of 
life to suppose that a form might become a fixed internal parasite, 
and, subsequently becoming freed from its host, attain to a higher 
grade of organisation. Yet this is what we must believe to hold good, 
if the current views of Mammalian development be accepted-as correct. 
From my standpoint, on the contrary, the larva may become a 
fixed internal parasite, and none the less there may arise upon it a 
more highly organised and, when fully developed, free-living form, the 
Mammal. Witness must be borne to the circumstance that MULLER, 
KLEINENBERG, and Kennex had already recognised that in some few 
divisions of the Invertebrata the mature form always arises upon a larya. 
In such groups as the Echinodermata an alternation of ge- 
nerations is now an obvious explanation of the facts, and when so 
magnificent an investigator as JOHANNES MÜLLER proved this nearly 
fifty years ago, one asks, in vain perhaps, why modern embryologists, 
like KorscHELT and Heer in their otherwise admirable „Ent- 
wickelungsgeschichte”, ignore it? 
The „Recapitulation Theory” and the mesoderm question 
have overshadowed the fact, and concealed the recognition of an alter- 
nation of generations. 
But the so-called “law of ontogeny” itself is no explanation 
of the riddles of embryology, —- at most the recapitulation hypothesis 
holds for the development of organs, not of organisms. So far as 
the facts are available Metazoan development appears to me to be 
by means of an alternation of generations ?), in that from the fertilised 
1) I need do no more at this juncture than mention RAuBER’s mem- 
brane and the trophoblast of the Mammalian embryo. 
2) It is not assumed that all the phenomena classified as ”alternations 
of generations“ are of the same nature. 
