426... THE AMERICAN NATURALIST. [VOL. XXXII. 
indicates that the mesoblastic pole cells of annelids and mollusks are 
to be regarded both historically and ontogenetically as derivatives of 
the archenteron, and that the rudimentary cells of Aricia and Spio 
are vestiges or ancestral reminiscences of such origin. 
A reéxamination of the cell lineage of a polyclade, Leptoplana, 
shows that, as in the annelid or gasteropod, all of the first three 
quartets of micromeres give rise to ectoblast, while the second 
quartet gives rise also to mesoblast, each cell of this quartet seg- 
menting off three ectoblast cells, and then delaminating a large 
mesoblast cell into the interior. The third quartet apparently gives 
rise to ectoblast alone, though the possibility of its producing meso- 
blast is not excluded. The four macromeres remaining give rise to 
the archenteron, as Lang describes, first dividing to form four basal 
cells (corresponding in origin and position with the four basal ento- 
meres of annelids and mollusks) and four much larger upper cells 
which correspond to the fourth quartet of micromeres in annelids 
and mollusks. The posterior of these cells always divides before 
the others, sometimes equally and symmetrically, as in Discocoelis 
(Lang), but more often unequally. The cells thus formed give rise 
to a part of the archenteron, and not, so far as can be determined, 
to mesoblast. 
These observations show that the mesoblast of polyclades is of 
ectoblastic origin, and they suggest that the origin of mesenchyme 
cells from the second (Unio, Crepidula) or third (Physa, Planorbis) 
quartets in gasteropods may be a vestige or ancestral reminiscence 
of the mesoblast formation in the polyclades. They suggest, further, 
that the mesoblast bands (entomesoblast) of annelids and mollusks 
may have been historically of later origin than the mesenchyme 
(ectomesoblast) —a view which harmonizes, broadly speaking, with 
that of Meyer—and that the two symmetrical entoblast cells into 
which the posterior member of the fourth quartet divides in the 
polyclade may represent the prototypes of the entomesoblasts of the 
annelids and gasteropods. 
Early Stages in the Development of Molgula. — Mr. Crampton 
briefly reviewed his observations on the early history of the egg in 
Molgula manhattensis as follows :' 
The author emphasized, the fact that development begins not with 
the cleavage or fertilization processes, but even before. From the 
origin of the primary odcyte until the final assumption of the adult 
1 Paper read before the New York Academy of Sciences, Biological Section, 
Dec. 13, 1897. 
