212 Edmund B. Wilson. 



able size variations. This accounts for certain discrepancies in 

 the figures, which represent blastomeres from many different eggs, 

 and possibly even from different species, though most of them 

 are from P. cceridea. The typical size-relation in this species, 

 from the eggs of a single female, are shown in Fig. loo, the 

 successive concentric outlines representing the entire egg, the J-2- 

 blastomere, M-blastomere, >^-macromere and >^-micromere. Dis- 

 tinct deviations from these mean volumes will be observed in the 

 figures. 



A. ANALYSIS OF THE FIRST QUARTET. 



I. General development of isolated micromeres of the first 

 quartet (^s -embryos) . 



As described in my preceding paper, the development of the 

 posterior micromere of this quartet (id) in Dentalium differs 

 from that of the others in being the only one to form an apical 

 organ. In Patella this is not the case, each micromere giving rise 

 to a closed ectoblastic structure, bearing at the posterior end a 

 group of active trochoblasts and at the anterior end an apical 

 organ (Figs. 28-29).^ 



The first cleavage is slightly unequal (sometimes nearly or 

 quite equal), (Figs. 22-24). I ^t first supposed the smaller cell 

 to be the primary trochoblast (i") since in the whole embryo 

 this cell gives the appearance of being slightly the smaller (in 

 Trochus this division is described as "nearly equal"). When, 

 however, the entire j4-blastomere segments in the calcium-free 

 water it may clearly be seen, at least in some cases, that the larger 

 cell is the lower one (i'), and I believe therefore the primary 

 trochoblast is slightly larger than its fellow in the normal devel- 

 opment. This is typically followed by an equal division of the 

 trochoblast, and an unequal division of the other cell, giving a 

 group that closely represents one quadrant of the first quartet in 



^This has not been proved in Putclla by isolation of all four of the micro- 

 meres from one t^^g (as was done in Dentalium') ; but among the numerous 

 larvae obtained of this type all that were closely examined possessed the api- 

 cal organ. 



