No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 55 



cells of the primary and larval mesoblast differs from the first. 

 The divisions of the primary mesoblasts are teloblastic, those 

 of the larval mesoblasts bear no such definite relation to the 

 embryonic axis, but are irregular. The primary mesoblast 

 cells adhere and act more like a mesothelium. The larval 

 mesoblast cells are mesenchymal in their lack of coordination. 

 This difference in the two kinds of mesoblast enables one to 

 distinguish them throughout. In a considerably later stage 

 (PI. V, Fig. 69) the primary mesoblast forms definite germ- 

 bands with terminal teloblasts. In front of the germ-bands lie 

 the elements of the larval mesoblast. 



At first sight there appears to be a considerable gap between 

 the stage of Fig. 69 and the succeeding stage of Fig. 73, but 

 the gap is more apparent than real, and is due to two changes. 

 First the eversion of the shell-gland and secondly the appear- 

 ance of the larval adductor muscle and myocytes. Even in 

 the stage of Fig. 69 the adductor muscle and myocytes are 

 foreshadowed ; thus a myocyte is seen stretching from the 

 oral plate to the entodermic sac. 



The position of the mass of the larval mesoblast cells marks 

 the position of the future adductor muscle. Moreover, and 

 this is a fact of some importance, the nuclei of these cells 

 resemble the nuclei of the early adductor muscle cells. Again, 

 the primary mesoblast is a compact fundament in the earlier 

 stage (Fig. 69) ; so is it in the latter stage. Here, however, 

 transverse sections are necessary for its demonstration ; two 

 of these are shown in Figs. JJ and yZ, taken in the planes 

 marked by the lines {']']) and {y^) in Fig. 73. The most pos- 

 terior section (78) shows the mesoderm teloblasts still plainly 

 recognizable ; they have, however, shifted their position to the 

 sides of the entodermic sac {cf. Fig. 69). The primary meso- 

 blast in front of the teloblasts is in the form of stout wings of 

 cells stretching on each side from the entodermic sac to the 

 walls of the body. In the place previously occupied by the 

 elements of the larval mesoblast we have the larval adductor 

 muscle and the myocites. When, in addition, the above men- 

 tioned similarity of nuclear structure is remembered, it is 

 impossible to resist the conclusion that the larval adductor 



