34 Edmund B. JVilson. 



the absence of any structure comparable with the foot, mantle- 

 folds, shell-gland, or mouth (unless the posterior opening can be 

 so considered) though all these structures are present in the 

 normal control embryos. The absence of an apical organ is 

 shown as in other series, by the two from which Figs. 33 and 40 

 are taken. 



I would not speak too positively before examining additional 

 material, for in some of the other series a few small cells appear 

 that may be of the same nature as those seen in the normal 

 embryos, though they are far less numerous; yet the foregoing 

 evidence is sufficient to create a strong presumption that Cramp- 

 ton's result was correct. Crampton showed due caution In guard- 

 ing against the conclusion, from his observations, that the polar 

 lobe "contains prelocallzed mesoblast material," being probably 

 Influenced by the fact that in Ilyanassa the lobe appears to be 

 composed mainly of deutoplasm. He only concluded "that the 

 presence of the yolk mass In the cell D may be the stimulus which 

 causes that cell to act differently from the other macromeres, A, 

 B and C" ('96, p. 14). I believe, however, the facts brought 

 forward In this paper render it probable that the polar lobe (and 

 hence the cell D) does in fact contain a specific kind of cytoplasm 

 which. If not actually "prelocallzed mesoblast-material" is f.he 

 direct and necessary antecedent of that material. 



IV. 



LOCALIZATION OF THE APICAL ORGAN AND ITS CORRELATION 

 WITH THE POST-TROCHAL REGION. 



The failure of the AB half-larva to produce an apical organ, 

 though wholly consistent with the history of the lobeless embryos, 

 was to me a surprising fact; for the development of this organ 

 in other forms Indicates that all of the four quadrants contribute 

 to Its formation ; and in point of fact I had found in Patella that 

 not only do both the AB and CD halves produce an apical organ, 

 but also any of the ^-embryos, and even any Isolated micromere 

 of the first quartet. I therefore turned with much Interest to a 

 more detailed examination of the localization of this organ in 

 Dentalium; and this Involved the inquiry whether the correla- 



