No. 3.] POD ARK E OBSCURA VERRILL. 429 



the larva, which could hardly have come from the feebly devel- 

 oped mesoderm bands at the posterior end of the body. The 

 same suggestion would apply to Pomatoceros, as described by 

 von Drasche (No. 7), where PI. XXXVII, Fig, 20, shows a 

 muscle cell just underneath the apical pole, and the small 

 compact mass of seven mesoblast cells, showing no trace of 

 differentiation, quite at the other end of the larva. 



The cell origin of the larval mesoblast in the forms where it 

 has been thus far described is as follows : 



Fourth Quartette. — At the 64-cell stage the fourth group of 

 micromeres has just been formed, and, so far as one can tell 

 with the microscope, are all exactly alike. Very soon, how- 

 ever, as we have seen, one of the cells divides bilaterally, thus 

 aiding in the establishment of the bilateral symmetry of the 

 body. These cells, like those of the third quartette just adjoin- 

 ing them, early show a tendency to invaginate, and they divide 

 at the same time as the latter, each budding off ventrally and 

 posteriorly a very small cell (PI. XL, Fig. 51). The two then 

 invaginate and can be seen through the transparent X-cells 

 lying just underneath the surface. It is important to notice, 

 however, that at the same time that these cells invaginate a 

 gastrulation begins, and the entoderm cells push in at the 

 same time as the cells 4di and 4d2. (Since until the stage of 

 PI. XL, Fig. 57, the mesoderm is not fully differentiated, I have 

 retained the designation 4di, etc., for the descendants of 4d in 

 the earlier figures. The small cells which enter into the arch- 

 enteric wall I have indicated by en in PI. XL, Figs. 51-57.) 

 This accounts for the fact that the descendants of 4d undergo 

 such extensive shiftings of position without at the same time 



1 Here, as elsewhere in the comparative portion of this paper, I have modified 

 the nomenclature to conform with that adopted for Podarke. (See also p. 465.) 



