72 C. 0. Whitman 



is a gap between Protozoa and Metazoa Jas now defiued ; and no one 

 could object to seeing it fiUed. Between the homoblastic and tbe diplo- 

 blastic 1 conditiou tbere is a distance that has not yet been measured ; 

 and I think few will deuy tliat it is immensely gveater thau that which 

 now separates the triploblastic from the diploblastic state. The nature 

 of this gap has been indieated by Gegenbaur, in placing the distinetion 

 between Protozoa and Metazoa^ not in the niimber of cells, but in t h e 

 arrangement of the same in distinct layers of different 

 functional value. From this point of view it is obvious that the 

 hiatus cau not be fiUed by such animals as the Dicyemids and Ortho- 

 nectids. 



Van Beneden denies the existence of a mesoderm in either of these 

 parasites, and on this ground alone attempts to justify the creation of 

 a new type. Even if this denial were well founded — and I believe it 

 is not — diploblastic animals could not bridge over the gulf between 

 Protozoa and Metazoa ; and if a new type be admitted for such animals, 

 what would be done in the event of the discovery of a Blastula? 



If diploblastic animals be deiined as animals composed of two 

 cell- layers, and triploblastic animals as having three distinct 

 cell-layers, the distinetion between the two would certainly be 

 great ; but the moment we insist on extending the meaning of the word 

 mesoderm so as to include the mesenchym and a geìatinous lamella 

 entirely free from cells , we virtually acknowledge that no broad line 

 of distinetion can be maintained. Now this is precisely what Van Be- 

 NEDEN has been compelled to do in order to bring bis views into harmony 

 with the conclusions reached by the Hertwigs. If, on the other band, 

 we adhere to the definition of the mesoderm as a cell-layer which arises 

 between the ectoderm and endoderm during the earlier stages of develop- 

 ment — not as the result of au ulterior histological dififerentiation — 

 then it becomes necessary to include both diploblastic and triploblastic 

 animals in the metazoic group , and the idea of a didermic Mesozoon is 

 left without »a locai habitation«. It appears to me then — even on the 

 supposition that these parasites are strictly diploblastic — there are no 

 adequate grounds for separating them from the Metazoa : and it is quite 

 impossible to allow that they represent forms »qui ont fait la transition 

 entre leu Protozoaires et les Métazoairesv^. 



But is there no trace of a mesoderm in the so-called Mesozoa ? I 



' This terminology is borrowed from Lankester. »On the Gemi. Layers« etc. 

 'Ann. Mag. Nat. Hist.' 1873. 



