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sing io, each buds off a single snaall, but by no means rudimentary 

 cell (e, e), toward the vegetative pole and into the endodermal mass 

 of cells. The right one of these two cells is perceptibly larger than 

 the left, so that the division renders the pole-cells (3£-M), from 

 which arise the right and left mesoblast bands, of equal size (Fig, 2). 



In view of the divergence of opinion in regard to the fate of the 

 small cells, i. e. those budded off from the mesoblast pole -cells in 

 Annelids and Molluscs, their ultimate destination has been followed 

 with great case in Thalassema, and I believe there can be no doubt 

 that they later form a part of the enteric wall. They, therefore, agree 

 in fate with the corresponding cells in Nereis and Aricia (Wilson), in 

 Crepidula (Conklin), in Podarke (Treadwell), and possibly Unio 

 (Lillie) and Dreissensia (Meisenheimer). On the other hand these 

 same cells are said to take no part in the formation of the enteric 

 wall and are described as "secondary mesoderm" in Umbrella (Hey- 

 MONs), Planorbis (Holmes), Amphitrite (Mead) and Arenicola (Child). 

 In Aplysia (Carazzi) there are several of these small cells, two of 

 which are entoblastic and the others mesoblastic. The corresponding 

 cells in Capitella are "paedomesoblasts" (Eisig). 



After budding off the entoblastic elements, the M cells, which 

 are now close together and towards the dorsal side, begin to separate 

 and migrate laterally. Soon, and exactly simultaneously, they divide 

 equally in a plane at right angles to the long axis of the trochophore. 

 There are now, accordingly, two mesoblast cells on each side. The 

 pressure of the rapidly dividing endodermal cells has shoved them 

 away from the posterior end of the embryo. The lateral divergence 

 continues until they reach their final position on the ventral side in 

 the fully developed trochophore. The mesoblast bands of the young 

 trochophore consist of not more than five cells each, all of nearly the 

 same size (Fig, 3, D). A somewhat similar separation of the mesoblast- 

 bands, according to Hatschek's description, is found in Criodrilus, 

 In Thalassema the bands lie on the inner side of the body wall of the 

 trochophore and not on the enteric wall. Meyer finds that in some 

 annelids the mesoblast bands lie as in Thalassema, whereas in others 

 they abut on the enteric wall. In Echiurus (Hatschek), where the 

 trochophore is quite similar to that of Thalassema, the bands occupy 

 a like position, but the pole cell at the posterior end is a much more 

 prominent feature in the former than in the latter. The long duration 

 of the free - swimming trochophore stage (at least six or seven days) 

 is probably accountable for the very meagre development of mesoblast 

 bands. In calling these rows of cells, mesoblast, I have not depended 



