No. I.] THE EMBRYOLOGY OF CREPIDULA. 149 



tral side ; this is due to the fact that they are carried down 

 over the yolk cells with the overgrowing ectoblast. When 

 first seen on the dorsal side, Fig. 42, etc., the teloblasts and 

 their bands lie anterior to the enteroblasts. After they have 

 moved around to the ventral side of the egg, with the general 

 overgrowth of the ectoblast, the teloblasts lie posterior to the 

 enteroblasts. Figs. 57 et seq., although preserving the same posi- 

 tion relative to the bands as before. This apparent change of 

 position can be readily understood by comparing Figs. 51-54, 

 in which an intermediate condition is shown. 



Even in the later stages the nuclei of these mesoblast cells 

 can be plainly seen just beneath the ectoblast, but it soon 

 becomes impossible to recognize the cell boundaries, especially 

 at the anterior ends of the bands. At the last stage in which 

 the entire bands can be seen, Fig. 53, there are eight or nine 

 cells in each. In older embryos the teloblasts and posterior 

 ends of the bands may remain distinct, while only scattered 

 nuclei can be recognized at the anterior ends, Figs. ^7 et seq.; 

 but ultimately the teloblasts and all traces of the bands disap- 

 pear. Figs. 74 et seq. 



2. The Scattered Mesoblast Cells. {Larval Mesoblast^ 



In no case can the cells derived from the bands be traced 

 anterior to the first, or transverse, cleavage furrow, nor over 

 the ventral face into the lips of the blastopore. Yet in all 

 stages from Fig. 60 onwards scattered mesoblast cells are 

 abundant at the anterior end of the embryo and over the whole 

 ventral surface, but particularly in the region of the blastopore. 

 It seemed impossible that so many cells so widely scattered 

 could have come in so short a time from the mesoblastic bands, 

 and I was therefore led to look for another source of this scat- 

 tered mesoblast, especially after the publication of Lillie's ('23) 

 beautiful results on the double origin of the mesoblast in Unio. 

 I was not able, however, to trace these scattered cells to their 

 source in the relatively small eggs of C. plana and C. fornicata; 

 but since the plates of this article were sent to the lithographer 

 I have found in the large eggs of C. convexa and C. adunca, of 



