54 CURRENT LITERATURE. 



Dr. Meisenheimer, who has already given us the results of an investigation on 

 the cell-lineage of the Pulmonata, taking Limax as an example, now deals with the 

 Lamellibranchia, basing his conclusions upon a detailed study of the cleavage 

 exhibited by the fertilized egg of Dreissensia, a particularly happy selection since 

 this form belongs to an undoubted lowly division viz. the Filibranchia. Dreissensia 

 is further interesting, for it alone of our fluviatile bivalves retains a free swimming 

 Trochophore larva, a fact which is usually interpreted as indicating a comparatively 

 recent incursion into freshwater, whereas in the other genera which have been simi- 

 larly investigated (Cyelas by Stauffacher, and Unio by Lillie and others), the stage 

 corresponding to this is almost wanting. 



Unfortunately the results to be derived from such an investigation as the above, 

 are hardly commensurate with the toil which it involves, since the results arrived at 

 by the now numerous investigators in this field are so uniform. We, consequently, 

 cannot help wishing that Dr. Meisenheimer had directed his attention and marvel- 

 lous patience towards some other group of animals less known, in this respect, than 

 the Mollusca. 



With regard to the conclusions deduced from a study of this nature, we think a 

 word of warning should be offered to those who have never attempted to trace back 

 the ontogenetic history of any adult organ beyond its first definite appearance as a 

 cell-complex, back to the earliest divisions of the egg, back even to one of the four 

 primary blastomeres. These zoologists will hardly realize the difficulties and uncer- 

 tainty which beset the attempts to identify a given cell in the different cleavage 

 stages, and consequently they will be inclined to accept without hesitation the 

 conclusion which investigators in this field put forward, their acceptation of these 

 deductions will be still more readily given, if they study the beautiful figures in the 

 monograph before us, where they will find each cell carefully lettered and its history 

 traced, as if its existence had been followed continuously in a single specimen. 

 This, however, is not the case, and when we consider the enormous difficulties in 

 homologizing these cells, each having to be identified anew at every stage, since it 

 is impossible to keep a live egg under observation for any time, it will be apparent 

 that these numbers merely signify a certain observer's interpretation of the cells at a 

 given stage, and consequently the conclusions cannot be unhesitatingly accepted, 

 their main value lying in the fact that for the most part they are in accord with 

 those of other workers in the same field of inquiry. 



Dr. Meisenheimer, as we have implied above, treats in great detail with the 

 cleavage of the fertilized egg, and the subsequent fate of the derived blastomeres, 

 and in this his observations differ in no vital respects from those of all former 

 workers. He then traces the formation of the Trochophore and its organs. Here 

 he is at variance in many points with most of the earlier observers, since he finds 

 that in Dreissensia, as m Limax, the primitive kidney is of ectodermal origin, and 

 his figures in this respect appear to be most conclusive. 



One of the most interesting observations from a phylogenetic standpoint, is to 

 be found in his account of the origin of the nervous system, where distinct pleural 

 ganglia are seen to arise in the embryo from the post-velar ectoderm, and therefore 

 quite distinct from the cerebral ganglia, with which, however, they eventually fuse, 

 since the latter arise as usual from the apical plate of the velum. 



A most radical change in our ideas will be necessary if Dr. Meisenheimer is 

 correct in his interpretation of the origin of the adult kidney and heart, for he would 

 derive these organs in Dreissensia as in Limax from the ectoderm, whereas all other 



