EMBRYOLOGY OF THALASSEMA MELLITA 223 



species of Polygorditis (Wilson, '91) and also in Podarke and 

 Thalassema. Although Hatschek's figures indicate that Eiipo- 

 matus forms an exception to this generalization, I believe that 

 this is due to his confusion of the ccelomesoblast and mesen- 

 chyme, as will be explained immediately, and that in this form, 

 too, there are, in the earlier stages at least, no actively budding 

 pole-cells. It is reasonable to suppose, in fact, that the presence 

 of teloblasts in many types is a secondary adaptation to a more 

 direct development ; a conclusion supported by the conspicuously 

 teloblastic character of the divisions in leeches and oligochsetes. 



2. The Ectoniesoblast. 



Heretofore the study of the cell-lineage of a number of anne- 

 lids and molluscs has shown that the so-called " larval mesen- 

 chyme " arises from certain ectodermal cleav^age cells of the 

 second or third quartet, and entirely independently of the 

 coelomesoblast. This is also notably the case in Thalassema 

 where ten large ectomesoblast cells sink in and give rise to all 

 the mesenchyme. Three of these cells are from the a, c and d 

 quadrants of the third quartet and seven from the first quartet 

 of ectomeres. The most important source of functional mesen- 

 chyme in Thalassema are the three cells from the third quartet, 

 namely, 3<^2. 2. 2. i» 3^^2.1.2.1. ^"^ 3(^2. 2. 2- The first two sink into 

 the cleavage-cavity, just before gastrulation (PI. II, Fig. 21), and 

 lie at first close to the two coelomesoblast cells. They soon 

 migrate laterally and bud' off simultaneously small cells toward 

 the M cells (PI. II, Fig. 24 ; Text-Fig. 8, E, F\ dividing like 

 teloblasts, but in the reverse of the ordinary direction. So close 

 is the connection of these cells with the coelomesoblast (Text- 

 Fig. 9, A^ B and (7) that one would certainly be led to think 

 that they formed a part of these bands unless their cytogeny 

 had been carefully followed. The condition here observed is 

 similar to that described by Treadwell in Podarke. The progeny 

 of these two cells forms almost the entire mesenchyme of the 



^ Conn's error in describing the mesenchyme as arising from the entoderm near the 

 blastopore is accounted for by the fact that these ectomesoblast cells lie in close asso- 

 ciation with the entomeres, and that he worked almost entirely on living material. 



