26 = Dr. W. Salensky on Héckel’s Gastrea Theory. 
lamella constitute a form which resembles the Planula-form 
of the Coelenterata and differs from this form only in certain 
animals by the further differentiation of the middle germ- 
lamella. The other form, from which the differentiation first 
commences, and which consequently is not to be compared to 
the Planula, I have called ‘ Blastula,” merely in order to 
indicate by this name that developmental state of some 
animals starting from which differentiation of the germ- 
lamelle occurs somewhat otherwise than in the Planula. We 
have met with this form in various animals, and_ briefly 
explained their further process of differentiation. ‘The sim- 
plest differentiation consists in that some cells of the Blastula 
begin to distinguish themselves from the rest. By this, two 
germ-lamelle are at once indicated, and the grade of organi- 
zation equivalent to that of the Planula is attained. The two 
germ-lamelle may be further developed in different ways: 
either they may constitute a body, which is the Gastrula (asin 
Amphioxus, the Ascidia, &c.) ; or the inner germ-lamella may 
be covered by the outer one, by which no Gastrula-form is pro- 
duced (as in the Insecta). 
In these brief remarks on the Gastrwa theory I have only 
desired to bring together the facts with which | endeavoured 
to clear up the significance of that theory for myself. ‘The 
negative result at which I arrived rests upon facts, especially on 
these—that the Gastrula is not of general occurrence, and that 
the embryological phenomena cannot be brought into causal 
connexion with this fundamental form. Even if the Gastrula 
were of as general occurrence as Hiickel states, this would by 
no means prove that it is truly an ontogenetic fundamental 
form; for what do we gain by the assumption that the 
Gastrula is a fundamental form of the development of all 
Metazoa, if we cannot by this form explain the differences 
in the development of nearly allied animals (e. g. Amphioxus 
and other Vertebrata, Ctenobranchiata and the other Proso- 
branchiata, &c.)? By the Gastrwa theory we cannot explain 
the difference in the development of Lumbricus and Euaxes. 
But very many such examples exist; and they show that, 
between animals standing near each other systematically, 
essential differences may occur in the foundation of their 
organs. This fact, however, appears so paradoxical only 
because we are now accustomed to deduce the relationship of 
animals only from anatomical facts, and to conclude from 
similarity of organization that there is similarity of develop- 
mental processes. But in order to ascertain the mutual rela- 
tions of organized forms, we should employ all the methods 
of natural history; we must regard the structure of the 
mature organic forms as the result of the ontogenetic pro- 
