328 RANDOLPH. [Vol. VII. 



The mesoderm just described seems to resemble the "migra- 

 tory mesoblast " of Wilson (25), and the "primary mesoderm" 

 of Meyer (14). This tissue is described by both observers as 

 arising in the embryo from a common foundation, but accord- 

 ing to Meyer it is distinct in origin from the great mass of the 

 mesoderm. It is true mesoderm, while the latter is trans- 

 formed reproductive tissue. From the "primary mesoderm" 

 Meyer derives the circular and the transverse muscles, the 

 muscles of setae, dissepiments, and mesenteries, as well as the 

 retroperitoneal connective tissue, and in some cases part of 

 the nephridia. 



In the regenerated tail of Lumbriculus these small cells give 

 rise to the circular muscles and to the wall of the dorsal blood- 

 vessel and its connections, and they arise separately from the 

 great mass of the mesoderm. The ventral mesentery, however, 

 as will be shown in the following description, appears to arise 

 solely from the neoblasts. 



2. The continued division of the neoblasts results in a com- 

 pact mass of embryonic cells that fill all the ventral space (Fig, 

 6). A little later this solid mass extends dorsally on each side 

 to the height of the dorsal setas (Fig. 8, x). A separation in the 

 new mesodermic tissue now takes place in the median dorso- 

 ventral line, and also at a little distance from it on each side, 

 giving rise to four groups of cells, a small pair on each side of 

 the median line and one larger mass on each side of these. 

 Very soon after this stage the smaller middle groups fuse, and 

 thus three cell masses are formed, twQ lateral and a composite 

 median mass (Figs. 6 and 7). Cavities soon appear, one in 

 each lateral mass and two in the median, corresponding to the 

 two masses that formed it. These two median cavities ulti- 

 mately coalesce and become the part of the coelom between 

 the ventral nerve cord and the ventral blood-vessel (Figs. 10 

 and 11). 



The differentiation of the mesoderm follows rapidly upon its 

 development. Each lateral element separates into the parietal 

 and visceral layer of its side. By continued cell division the 

 parietal layer remains of considerable thickness ; but the visceral 

 layer becomes gradually more and more extended in consequence 

 of the increase of the worm in length. From the parietal layer 

 are developed the longitudinal muscles in their well-known dis- 



