360 PROCEEDINGS OF THE ACADEMY OF [April, 
very wisely done for Physa. Robert (1903), in his excellent paper on 
the development of Trochus, which 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 likewise 
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 larve 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 Toénniges (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 Mollusks. 
In Dinophilus (the cleavage of which is, from work now being done 
in this laboratory by Dr. J. A. Nelson, typically annelidan in character) 
Schimkewitsh (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 Annelid 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” (7.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 third 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 Hisig (1898) for Capitella differs widely from 
the results of most workers on annelidan and molluscan embryology. 
