Experimental Studies on Germinal Localization. 41 



A considerable proportion of these embryos fail to develop 

 into larvae, breaking up sooner or later Into loose groups of 

 cells that perish. Many, however, develop Into actively swim- 

 ming larvae, but these, whether large or small, are never normal 

 trochophores. While showing many variations, and often being 

 more or less Irregular In form, these larvae tend In general 

 towards, and sometimes agree precisely with those derived from 

 whole eggs minus the polar lobe, from the AB half, or the A, B 

 or C quarters (Figs. 70, 86). They are in general more or 

 less distinctly pyriform, swimming actively by the long cilia that 

 are more or less irregularly disposed about the posterior enlarged 

 region. A typical case is shown in Fig. 70 (from a preparation, 

 the cilia from the living larva) produced from the upper two- 

 thirds of an egg after exactly horizontal section. The cleavage 

 of this fragment was similar to that shown In Fig. 69. This 

 larva Is In every respect closely similar to the lobeless larva, though 

 the pre-trochal region is more expanded than usual, forming 

 a large hollow vesicle enclosing a few loose cells, and with a 

 slight thickening at the anterior pole, but without anything like 

 a true apical organ. The posterior region Is filled with a crowded 

 mass of rounded cells. Transverse sections of this larva show that 

 this mass Incloses a very small central cavity; but It is Impossible 

 to determine whether mesoblast cells are present or not. In a 

 very few cases an apical organ Is present in such larvae; but this 

 is so rare that I attribute Its occasional presence to the fact that 

 the plane of section was not quite correctly determined, a portion 

 of the lower polar area having been in fact included in the piece. 

 Another possibility is that the specific material of the polar lobe 

 extends so far up into the interior as to be removed by a section 

 that externally passes quite outside the polar area. This Inter- 

 pretation is supported by the fact that In a very few cases, when 

 the upper fragment Is considerably larger than the lower one, I 

 have seen the upper fragment form a very small polar lobe. 



The development of the lower fragment — i. e., one that In- 

 cludes the lower polar area — differs In a remarkable way from 

 that of the upper one, both In the form of cleavage and in the 

 end-result. Whether obtained by horizontal or oblique sections. 



