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the same time the two primitive mesoderm cells have moved 

 into the inside, having- detached themselves from their con- 

 nection with the other cells. In the lateral aspects seen in 

 Fig. 117 A and B, only one of the two cells can be recognized. 

 It should be mentioned here that later they divide (Fig. 

 118). The two primitive cells still continue to be dis- 

 tinguished from the newly formed ones by their greater size. 

 Hatschek calls them the two pole cells of the mesoderm. 

 They lie at the ends of the two mesodermal bands formed 

 by cell-proliferation. In the farther development of the 



Pio. 118. — A and B, trochophores of Euponiatus in younger and older stages 

 of development (after Hatschek). a, anal opening ; ab, anal vesicle ; fcn, head 

 kidney; in, mouth-opening; mes, mesodermal bands; mit, muscles; oc, eye-spot; 

 ot, auditory vesicle ; sp, apical plate. 



larva the intestine bends toward the anal side, in order 

 to fuse in later stages of development with the originally 

 slight depression of the ectoderm, which produces the hind- 

 gut and anus (Fig. 118). Some time before the completion 

 of this process, the blastopore had become considerably 

 narrowed. It assumed the form of a fissure which closed 

 (from behind forwards) and left only a small opening re- 

 maining in front. At the place of this last trace of the 



