1904,] NATURAL SCIENCES OF PHILADELPHIA. 359 
In 1897 Wierzejski showed that in the sinistral Pulmonate Physa sec- 
ondary mesoblast arises from certain derivatives of the third quartet 
(3c and 3b), and similar conclusions were reached in the same year for 
Planorbis by Holmes, 3c and 3b here also giving rise to cells which sink 
into the segmentation cavity. 
~ Lhe formation of the secondary mesoderm in Fiona is strikingly 
similar to its manner of origin in Planorbis, as described by Holmes. 
The following diagram (text-figure 2), showing the cleavage history of 
the ectomesomeres of the two forms, indicates how close a comparison 
is possible. | 
g Q2 & Oe Go SS Oe 
D 68 98 ¢ 
@ 
@ 28 Be 
G8 GS oe 29 
Fig. 2.—Diagrams showing the manner Ge formation of secondary mesoderm 
in (a) Planorbis (Holmes) and (6) Physa (Wierzejski) and Fiona. The cells 
2ontaining secondary mesoderm are stippled. 


It will be noted that four cells of each anterior quadrant are meso- 
dermal in Planorbis, while in Fiona only two have this fate, the 
smaller cells, 3a, ??, and 3b”, 2, of Fiona remaining in the ecto- 
derm. For Physa Wierzejski came to similar conclusions, but here 
there is even closer correspondence, for the cells 3b7!”, 7 and 3c7!”, 
of Physa remain in the ectoderm exactly as they doin Fiona. Accord- 
ing to the nomenclature used by these two investigators secondary 
mesoblast arises from quadrant B and C, while in the dextrally cleaving 
egg of Fiona it comes from quadrant Aand B. Holmes and Wierzejski 
have attempted to use the same sequence of lettering for sinistral 
forms as that commonly employed for the dextral, and have thus been 
led into error, Holmes particularly arguing for a non-homology of 
cells upon this score. When the dextral or clock-wise sequence is 
employed for a sinistral form this difference in designation necessarily 
results if the cell which is to give rise to the entomesoblast be labelled 
D. The more natural and logical method is to label the cells of a 
sinistral form in an anti-clock-wise sequence, as Crampton (1894) has 
