428 TREADIVELL. [Vol. XVII. 



usual position in the segmentation cavity, and they would 

 undoubtedly be described as such by any one who saw merely 

 the preparation figured in PI. XL, Fig. 54. Very soon, how- 

 ever, the larval mesoblast cells begin to migrate {PI. XL, Figs. 

 53, 55, 56). In PI. XL, Fig. 56, is shown the position of the 

 posterior larval mesoblast, while the anterior, at this stage, is 

 composed of four cells on a side. The cells elongate and 

 become the larval musculature. They are especially well 

 developed in the region of the prototroch, under which the 

 long, spindle-shaped cells may easily be recognized in later 

 stages. (See Text-Fig. 8.) 



Comparative. — A larval mesoblast was first discovered by 

 Lillie (No. 21) in Unio, where it arises asymmetrically in the 

 A quadrant only, from the second group of micromeres, 2a2 +. 

 This migrates so as to lie symmetrically in the cleavage cavity, 

 and by its division are formed cells which become metamor- 

 phosed into the "myocites" and larval adductor muscles, 

 which are functional only during larval life. Later Conklin 

 (No. 5, a), in Crepidula, found larval mesoblast arising from 

 the second quartette in three quadrants. A, B, and C. In the 

 left-wound gasteropods, Physa and Planorbis, which have the 

 "reversed" type of cleavage, Wierzejksi (No. 31) and Holmes 

 (No. 15, b) found it arising from the third quartette in B and C 

 quadrants ; in these reversed cleavages, therefore, symmetri- 

 cally arranged. In annelids, thus far, the structure in ques- 

 tion has been found only in Aricia, Capitella, and Podarke. In 

 Aricia, Wilson (No. 34, g) finds two cells arising symmetrically 

 from either the second or third quartette. Through lack of 

 material he was unable to discover its exact origin. In Capi- 

 tella, Eisig (No. 8) finds it arising from the ventral products 

 of the second division of 4d, thus corresponding in origin to 

 cells which in other annelids become entoderm. (See p. 451.) 

 To this point I shall return later. 



From the figures which Hatschek gives for Eupomatus, it 

 seems to me extremely probable that a larval mesoblast is pres- 

 ent there also, although the point is not one to which Hatschek 

 paid any attention. His figures (PL XXXIX, Fig. 42 to PI. XL, 

 Fig. 49) show scattered muscle cells in the upper hemisphere of 



