Dr. W. Salensky on Héckel’s Gastrea Theory. 21 
lamella; in Hydrophilus exactly the same process occurs in 
the formation of the germ-lamelle. In the Ascidia the 
intestino-glandular lamella and the middle germ-lamella 
are formed from the inferior germ-lamella; in Hydrophilus 
the same differentiation takes place in the inferior germ- 
Jamella. 
The two forms in which the process of differentiation in the 
germ-lamelle commences, (namely 1, the planula, in which 
the two germ-lamelle are already differentiated, and 2, the 
blastula, in which an indifferent cellular layer, afterwards 
becoming differentiated, is formed) appear to pass into each 
other. It is to be hoped that such transitions will hereafter 
be made known in greater numbers; the development of the 
Vampanularie trom the ova of EHucope polystyla* may at 
present serve as an example. The ova ot this Medusa pass 
through a regular process of segmentation, which leads to the 
evolution of a Blastula-stage. This latter form subsequently 
passes into the Planula in this way: in the interior of the 
blastula the cells of the inferior germ-lamella (entoderm) are 
formed; and these accumulate more and more, until finally they 
entirely fill the cavity of the Blastula. By this mode of deve- 
lopment there is produced from the Blastula a form which 
consists of two germ-lamelle and possesses no cavity in its 
interior, 7. ¢. a Planula-form. Palemon also presents a similar 
transition into the Planula-form in its development; but in it 
an invagination is formed before the transformation takes 
place. ‘The stage with the invagination may have a great 
resemblance to the Gastrula-stage ; but it is essentially dis- 
tinguished therefrom by the circumstance that the invaginated 
part in Palemon does not form the entoderm, as is the case 
in other true Gastrula-forms, but always remains exoderm. 
The transitions just indicated may, to a certain extent, 
explain the mutual relations of the Planula and the Blastula. 
The Planula-form occurs most frequently in the ontogeny of 
animals; and for this reason it may be regarded as the funda- 
mental form. The cases in which the Blastula passes into 
the Planula appear still further to support this assertion. 
The other cases in which (as, for example, in Amphiorus, the 
Ascidia, &c.) a Gastrula originates from the blastula, are 
united even by the Blastula-stage with the case of Hucope, 
and differ from the latter case by the circumstance that they 
lead very soon to the development of the intestine; here, 
therefore, the Planula-form (which, as is well known, pos- 
* Kowalevsky, ‘ Beobachtungen iiber die Entwickelungsgeschichte der 
Celenteraten’ (Ruasian). 
