IX 



MOLLUSCA 



331 



seen. The conclusion is therefore inevitable that the first polar lobe 

 contains the material necessary not only for the formation of the 

 whole post-trochal region, but also for the formation of the apical 

 plate. It must also contain the material necessary for the formation 

 of the mesodermic bands. 



This conclusion is confirmed by separation of the first two blasto- 

 meres ; both of these, when isolated, continue to segment as if they 



Fig, 261. — Larvae resulting from the development of eggs of Dentalhtm from wMcli the 

 polar lobe has been removed. (After Wilson.) 



Aj larva, twenty-four hours old, developed from egg from which the first polar lobe has been re- 

 moved. B, Larva, twenty-four hours old, developed from egg from which the second jjolar lobe had 

 been removed. Letters as in Fig. 258. . 



'^ptr 



formed part of a whole egg, but both subsequently give rise to larvae 

 which swim about, though they possess a confused irregular proto- 

 troch. The larva derived from AB, however, in its general structure, 

 resembles the larva developed from a wliole egg from which the first 

 polar lobe has been cut off, because 

 it has neither apical plate nor post- 

 trochal region. The larva derived 

 from CD, on the other hand, which 

 carries the polar lobe, though it is 

 asymmetrical and has too small a 

 pre-trochal region, has too large a 

 post-trochal one and possesses a 

 well-defined apical plate. 



If the egg be allowed to reach 

 the 4-cell stage, and if the polar 

 lobe that is then protruded, that is 

 the second one, be removed, a larva 

 is produced in most respects similar 

 to the one which arises from an egg 

 from which the first polar lobe is 

 removed; it possesses neither meso- 

 dermic bands nor post-trochal region, but it possesses an apical plate 

 and the characteristic apical tuft of cilia. Therefore the second polar 



Fig. '262. — Vertical section of a larva of 

 Dentalium developed from egg from 

 which the first polar lobe had been 

 removed, to show the absence of 

 mesoderm. (After Wilson.) 

 e-nd, protruding plug of endoderin. 



