432 TREADWELL. [Vol. XVII. 



which mesoderm can arise, unless we assume an ectodermal 

 origin, which is not probable. Hence I have no hesitation in 

 saying positively that they are the germ bands. 



The second pair of small cells, like the first, enter into the 

 wall of the archenteron. 



Comparative. — In all annelids, gasteropods, and lamelli- 

 branchs thus far studied, with one exception, the definitive 

 mesoblast arises from 4d. The exception is Capitella, where, 

 according to Eisig, 4d contains only a little larval mesoblast, 

 while the greater part of th£ cell is ectodermal and enters into 

 the ventral plate. The definitive mesoblast in this form arises 

 from 3c and 3d. (See p. 451.) In Nereis, Wilson (No. 34, d) 

 described small cells budded off ventrally from 4di and 4d2, 

 which he at first called "secondary mesoblast," and supposed 

 they lay on the dorsal wall of the archenteron. In Aricia 

 only one of these cells appears on either side. Later, how- 

 ever, Wilson has shown (No. 34, g) that the so-called second- 

 ary mesoderm cells do not form mesoderm at all, but really 

 become a part of the wall of the archenteron. In Amphitrite 

 (No. 22), Polymnia ^ (No. 34, d), and Arenicola (No. 4) this 

 entodermal portion is lacking in the cell 4d. In Amphitrite, 

 Mead described a very small cell at the anterior end of the 

 germ band, which he suggests may be a reminiscence of the 

 superficial budding which takes place at the surface in other 

 forms, and Wilson has definitely homologized the two sets of 

 cells. Inasmuch, however, as these cells always remain in the 

 mesoderm, such a comparison seems to me doubtful, and, if ves- 

 tigial at all, they represent rather some mesodermal structure. 

 As I have pointed out on p. 472, the small size of cells when 

 first formed is no proof of their vestigial character, since they 

 may subsequently undergo any amount of growth. In Clyme- 

 nella, Mead (No. 22) described the first division of 4di and 4d2 

 as equal instead of the very unequal division of Nereis or Aricia. 

 Wilson (No. 34, g) has suggested that the posterior portion of 

 each of these cells is entoderm. While this is possible, it has 

 not been proved, and hence the structure is of little value for 

 comparison. 



^ See, however, Wilson (No. 34, g), p. 12 of reprint. 



