226 TORREY 



tive annelids and molluscs have demonstrated, in recent years, 

 that a part of the mesoblast in these groups arises from the 

 ectoblast. This mesoblast, however, has commonly, and as I 

 believe erroneously, been considered to be purely larval and 

 transitory. In certain instances it has been possible to deter- 

 mine the blastomeres which give rise to the ectomesoblast, but 

 in many others merely the general regions. Nevertheless in 

 all the source has been either the second or the third quartet 

 of ectomeres. In the first class belong Unio (2<^2. i+5 Lillie, 

 '95) ; Crepidida (2a, 2b, 2c ; Conklin, '97) ; Physa (3^, y ; 

 Wierzezski, '97) ; Planorbis {'^b^, y^-, Holmes, '00) ; and 

 Podarke (3^2. 2.2* 3'^2. 1. 2. 3^2.2.2 5 Treadwell, '01). In the 

 second class we may place Alicia (Wilson, '89), Dreissensia 

 (Meisenheimer, '00), Cyclas (Ziegler, '85), Pisidiiim (Lankester, 

 '75), PJiolas (Sigerfoos, '95), Patella (Patten, '86), Pahidina 

 (Erlanger, '91). The figures given by Hatschek ('81) for 

 Teredo, Goette ('91) and Schierholz i^J^) for Anodonta, and 

 Horst ('82) for Ostrea also indicate, as Holmes ('00) points out, 

 that the mesenchyme in these forms must have originated from 

 the ectomeres. A study of Hatschek's ('85) figures of Eupo- 

 matus has led me to the conclusion that his description of the 

 origin of the mesenchyme is founded on errors in observation, 

 due to his working entirely on living material, and that in reality 

 a part, at least, of the mesenchyme has exactly the same origin 

 as in Tlialasse?na, e. g., from y and 3</. Hatschek described 

 the ccelomesoblast pole-cells as budding off mesenchyme before 

 giving rise to the ccelomesoblast bands, and, in fact, if the cell 

 lineage had not been followed carefully in Thalassema and 

 Podarke, the close proximity of the two forms of mesoblast 

 would have led one into a similar error. The striking similarity 

 in the origin of the ectomesoblast in these two forms justifies 

 us, I believe, in supposing that we may have the same condition 

 of affairs in Eupoutatus where the cleavage is also ''equal." 

 Meyer ('01) has also expressed himself as sceptical in regard to 

 the accuracy of the observations of Hatschek in this case and 

 calls attention to the fact that he neither figures nor appears to 

 have seen a single division of the protoblasts of the mesoblast. 



