No. 3-] 



PO DARKE OBSCURA VERRILL. 



431 



^ml 



^/' 



direction, at right angles to the prototroch, and eventually lie 

 on either side the ventral surface, just underneath the adoral 

 zone of cilia (PI. XL, Fig. 59). This is the latest stage to 

 which I have carried ? 



the mesoderm cells 

 in what I am posi- 

 tive were normal 

 embryos. In older 

 embryos I have 

 found a large pole 

 cell occupying a 

 position like the 

 posterior one of the 

 two cells figured 

 in Fig. 59, while a 

 row of small cells 

 extended anteriorly 

 from it. The diffi- 

 culties experienced 

 in getting larvae to 

 develop after the third day 

 have as yet prevented me 

 from securing any complete 

 details of the later history 

 of these cells, and since the 

 study of the trochophore 

 development has been taken 

 up by another worker, I 

 have thought it best to leave 

 the subject at this point. In ^^"' 

 calling these cells the germ 

 bands, I am not relying en- 

 tirely on analogy with other 

 annelids, although the re- 

 semblance is close, but on 

 the fact that both ectoderm 

 and entoderm are fully differentiated at this stage, and these 

 cells, lying in the segmentation cavity, are the only source from 



Me 



Fig. 7. — Horizontal optical section through Fig. 52, PI. XL, taken 

 at level of larval mesoblast. Imr, Iml, Imm, right, left, and 

 median larval mesoblast. Other letters as before. 



Gf? 



Fig. 8. — Sagittal section through stage of Fig. 57, 

 PI. XL. ec, ectodermal cell; IM, definitive meso- 

 blast, in its first division after eliminating all the 

 entodermal elements ; stoin, stomodaeum. 



