18 IIVLSOX. 



cell of the third quartet formed about the same time (Fig. 6, 

 A)} The second division is nearh' or quite horizontal, separat- 

 ing a second ectoblast-cell ("2'") directly above the original 

 or stem-cell (Fig. 6, B). The third ectoblast cell ('' 2^ "), which 

 is very small, is budded forth at the lower tip in the angle be- 

 tween the macromeres (Fig. 6, C, D). The three cells thus 

 formed (2\ 2^, 2^, Fig. 6) enter, as I believe, into the general 

 ectoblast. At the fourth division the stem-cell divides unequally 

 in a direction parallel to the surface, a large inner cell being de- 

 laminated off from a smaller superficial cell (2^, Fig. 6, D). 

 TJic inner ail is forced into the angle betzi'cen the two adjoining 

 '' macromeres y and forms one quadrant of the mesoblast ; tJic 

 outer cell flattens out at tlie surface ami is, I believe, afi ectoblast- 

 cell, though I am not entirely sure that it may not ultimately 

 migrate into the interior to form mesoblast. The four primaiy 

 mesoblast-cells thus formed rapidly multiply to form four 

 groups of rounded granular cells (Fig. 6, F) which may easily 

 be seen for a long time through the transparent ectoblast and 

 from which the greater part, if not all, of the adult mesoblast 

 is derived. 



It is clear from these facts that the cells of the second quartet 

 in the polyclade (/. e., in Leptopland) are not purely mesoblastic, 

 but are mesectoblasts. It seems equally clear that the formation 

 of " larval mesenchyme" from certain cells of the second quartet 

 in Unio and Crepidula must be regarded as an ancestral remi- 

 niscence or survival of the process that occurs in all four of the 

 cells in the polyclade, and it is an interesting question whether 

 such a survival may not also occur in the embryos of annelids. 

 A careful re -examination of Nereis with respect to this point has 

 thus far yielded a negative result. In Aricia, on the other hand, 

 it is probable that two mesoblast-cells arise from either the 

 second or third quartet, though the material at my command 

 has not enabled me to reach a decisive result. At the stage shown 

 in Figs. I, C, and 2, A, two large and very conspicuous rounded 

 cells are found lying, one on either side, in the cleavage-cavity 

 between the lateral ectoblast and the mesoblast-band (j', y, Fig. 



* Lang figures this division — PI. 35, Fig. 5. 



