242 Edmund B. Wilson. 



of six trochoblasts. Four of these, bearing powerful rows of 

 cilia, are evidently primary trochoblasts; two, lying in front of the 

 last, are much smaller, and are probably secondary trochoblasts 

 from the first quartet. Those of the second quartet have either 

 failed to develop or have broken away from their connections (as 

 very often occurs with all the trochoblasts owing to their ac- 

 tivity). No apical organ was seen in this larva; but I observed 

 an apical organ in several less normally developed individuals, 

 and since the apical organ constantly appears in the J/g-micromere 

 larva, there can be no doubt that it may appear also in the J4- 

 larvae. 



Perhaps the most interesting question presented by these larvas 

 is whether the AB and the CD half-larvae differ in respect to the 

 mesoblast; for if the mosaic principle holds for this structure, 

 one should expect to find coelomesoblast only in the CD half. 

 For the present I can give no certain answer to this question, fur- 

 ther than to state that in DentaJium the two larvae certainly dif- 

 fer to some extent in respect to the mesoblast, and there is possibly 

 some reason to conclude that they do also in Patella. 



In the latter form some of the larvae show a large rounded cell 

 in the upper region of the central mass (the dotted outline in 

 Figs. 113 and 115) which does not appear in others; and this 

 difference distinctly appears between the two twin larvae shown 

 in Figs. 113 and 114. This cell is possibly the primary meso- 

 blast, 4d; but it may also represent the large rounded cell which 

 I have considered to be 4D In the normal larva (Figs. 15-16). 

 This evidence, unsatisfactory as It is. Is mentioned as an Indica- 

 tion that the Internal structure of the two half-larvae shows dis- 

 tinct differences In Patella. In Dentalium the evidence is some- 

 what better, but still far from adequate, owing to paucity of ma- 

 terial and the confused appearance of the inner cell-mass, as seen 

 either In total preparation or In sections. Sections of the CD 

 larvae nevertheless show groups of smaller and irregular cells 

 lying between the large entoblast-cells and the ectoblast, and 

 there is little doubt that these represent in part the coelomeso- 

 blast. Sections of the AB larvae are in general closely similar to 

 those of the lobeless larvae described in my preceding paper, but 



