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 

 Planorhis by Holmes, 3c and 3b here also giving rise to cells which sink 

 into the segmentation cavity. 



The formation of the secondary mesoderm in Fiona is strikingly 

 siinilar to its manner of origin in Planorhis, 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. 



(©(S) 



03 00 



Fig. 2. — Diagrams showing the manner of formation of secondary mesoderm 

 in (at) Planorhis (Holmes) and (6) Physa (Wierzejski) and Fiona. The cells 

 3ontaining secondary mesoderm are stippled. 



It will be noted that four cells of each anterior quadrant are meso- 

 dermal in Planorhis, while in Fiona only two have this fate, the 

 smaller cells, 3a^^^, ^^^, and 3b^"", ^^^^, of Fiona remaining in the ecto- 

 derm. For Physa Wierzejski came to similar conclusions, but here 

 there is even closer correspondence, for the cells 3b^"^, ^^^^ and 3c^^", ^^^^ 

 of Physa remain in the ectoderm exactly as they do in 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 A and 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 



