360 PROCEEDINGS OF THE ACADEMY OF [April, 



very -wdsely done for Physa. Robert (1903), in his excellent paper on 

 the development of Trochus, wliieh has just reached this laboratory, 

 calls attention to the above and confirms opinions which had already 

 been embodied in this paper. Animals which are sinistral, or reversed 

 in their larval and adult stages, develop from eggs which are hkewise 

 reversed in their cleavage, and the designation of the blastomeres of 

 the egg should coincide with the condition of the adult, if any homology 

 of cells exists. The eggs of sinistral Gastropods have probably at an 

 early stage in their ovarian development undergone complete cyto- 

 plasmic and nuclear inversion, for only by such a process can the 

 reversed condition of the larvse and adults be understood or the reversal 

 of direction of the cleavage spindles be explained, and if such an inver- 

 sion be postulated, corresponding reversal of sequence in nomencla- 

 ture must ensue. 



Meissenheimer (1901) describes in Dreissensia a cell lying in the 

 cleavage cavity just under the First Somatoblast derivatives, but 

 which, he says, does not come from this group, though he is sure it is 

 of ectodermal origin. It later divides and forms muscle fibers. Simi- 

 lar conditions appear to be present in Cyclas (Zeigler, 1885). In the 

 fresh-water Prosobranch Paludina teloblastic pole cells are not found. 

 Scattered mesenchyme cells occur, and Tonniges (1896) states that these 

 have been produced from cells which lie in front of the blastopore. 

 If this be the case, the formation of mesoderm in Paludina is similar 

 to that of the secondary mesoderm of other MoUusks. 



In Dinophilus (the cleavage of which is, from work now being done 

 in this laboratory by Dr. J. A. Nelson, typically annehdan in character) 

 SchimkeTsitsh (1895) appears to have recognized ecto-mesoblast, for 

 he says: " Gleichzeitig (with the proliferation of Urmesodermzellen) 

 aber findet auch eine Immigration der Ectodermzellen in der Vorder- 

 theil des Embryos statt, und es wird durch diese Zellen eine Mesem- 

 chymanlage gebildet". 



In the Annehd Aricia, Wilson (1897) discovered mesoderm arising 

 from the two posterior quadrants which could not be derived from the 

 pole cells, and which he located as coming from "either the second or 

 third quartet" {i.e., from c^ and d^ or from c^ and c^). These conclu- 

 sions were strengthened by a preliminary account of Treadwell (1897) 

 on the cell lineage of Podarke, in which he derives secondary mesoblast 

 from the tliird quartet (3a, 3c and 3d), and these results are confirmed 

 in a later and more elaborate paper (1901). The account of the meso- 

 derm formation given by Eisig (1898) for Capitella differs widely from 

 the results of most workers on annelidan and molluscan embryology. 



