Dr. W. Siilensky on HdckeVa Gastraea Theory. 21 



lamella; in Hydrojyhilus exactly the same process occurs in 

 the formation of the germ-lamcll». 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 iu the inferior germ- 

 lamella. 



The two forms in which the process of differentiation in the 

 germ-lamcllffi commences, (namely 1, the planula^ in which 

 the two germ-lamellai are already differentiated, and 2, the 

 hlastulaj 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 

 Camjyanularue from the ova of Eucope polystyla'^ tnay at 

 present serve as an example. The ova of 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-lamella3 and possesses no cavity in its 

 interior, i. e. a Planula-form. Palcemon also presents a similar 

 transition into the Planula-iorm. 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-sta,ge ; but it is essentially dis- 

 tinguished therefrom by the circumstance that the invaginated 

 part in Pakemon does not form the entoderm, as is the case 

 in other true Gastrula-ioxvas,^ 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-ioxva. 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 Amphioxus, the 

 Ascidia, &c.) a Gastrula originates from the Blastula^ ai-e 

 united even by the Blastula-stage with the case of Eucope^ 

 and differ from the latter case by the circumstance that they 

 lead very soon to the development of the intestine ; here, 

 therefore, the Planula-ioxm (which, as is well known, pos- 



* Kowalevsky, ' Beobachtungen viber die Entwickelungsgeschichte der 

 Ccelenteraten ' (Russian). 



