Experimental Studies on Germinal Localization. 27 



organ. Sections show that these larvae have formed no shell- 

 gland, no mantle-fold, and apparently also no mouth. 



The foregoing account applies to the great majority of the 

 lobeless larvae; but occasionally an apparent exception occurs, the 

 careful examination of which only serves to confirm the rule. 

 In these exceptional cases a more or less reduced post-trochal 

 region appears to be present, and one individual was obtained 

 that in life seemed to possess this region in a fully developed con- 

 dition. Sections of these embryos show, however, that what 

 appears to be a post-trochal region is in reality a plug of ento- 

 blast cells, projecting through the blastopore-region, that arises 

 through defective gastrulation (Fig. 33). Such embryos some- 

 times show towards the upper pole a much larger cleavage-cavity 

 than in the normal form, — obviously a result of the failure of 

 the entoblast-cells to invaginate completely. This is conspicuously 

 shown in the larva, referred to above, which appeared to have a 

 fully developed post-trochal region. This larva, cut into longi- 

 tudinal serial sections, shows very clearly the failure of the ento- 

 blast-cells to invaginate properly, a large space being left in the 

 upper hemisphere above the archenteron. For this very reason 

 this larva showed very clearly, both as a total preparation and 

 after sectioning, the entire lack of an apical organ. 



The foregoing observations fully establish the conclusion, I 

 believe, that the material of the polar lobe is indispensable for the 

 formation of the post-trochal region and the apical organ, and as 

 shown beyond they give considerable reason for extending this 

 conclusion also to the ccelomesoblast. . That the failure to produce 

 a normal larva is not due to the lack of sufficient material, is con- 

 clusively shown by several additional facts. First, in Patella 

 the D-quadrant is no larger than the others, yet a post-trochal 

 region is formed that is relatively as large as in Dentalium. 

 Second, as will be described in Part V, much smaller larvae, pos- 

 sessing all of the typical parts, may be produced from fertilized 

 egg-fragments. Third, the same conclusion is afforded by the 

 history of isolated blastomeres, which also fully corroborates the 

 results obtained by removing the polar lobe from an entire egg. 

 If in the 2-cell stage the two blastomeres, AB and CD, be sep- 



