Experimental Studies on Germinal Localization. 23 



24) . The embryos gastrulate and develop with great regularity 

 into larvae that swim in the same characteristic progressive spiral 

 course as that of the normal ones. These larvae (Fig. 32) differ 

 from the normal ones in two obvious respects, namely, ( i ) the 

 post-trochal region is absent, or represented only by a smoothly 

 rounded surface from which no outgrowth takes place, and (2) 

 they show no trace of an apical organ. . The first of these results 

 fully accords with expectation; for studies in cell-lineage have 

 shown, both in annelids and in mollusks, that in forms possessing 

 a typical trochophore larva the ectoblast and mesoblast of the post- 

 trochal region are mainly derived from the two somatoblasts, 

 and I have shown that the first of these cells is certainly and the 

 second probably, derived mainly from the polar lobe (or lower 

 white area). The second result, on the other hand, is astonish- 

 ing, since the region that has been removed is diametrically oppo- 

 site to that from which the apical organ develops; but a large 

 number of operations have not shown one exception in this re- 

 spect and the most convincing corroborative evidence is afforded 

 by other experiments presently to be described. 



The structure and subsequent history of these larvae is very 

 widely different from that of the normal forms. As the cleavage 

 advances the symmetrical cells of the second and third quartets 

 close in around the lower pole, frequently followed in greater 

 or less degree by the cells of the prototroch; and after the gas- 

 trulation this region (the posterior region of the larva) becomes 

 somewhat expanded, so that the larva assumes a pyriform shape, 

 actvely swimming with the narrower end in front, and rotating 

 from right to left like a normal larva. The narrower anterior 

 region is uniformly covered with fine vibratile cilia which are 

 slightly longer near the anterior pole (as in a normal larva — 

 Fig. 32) ; but an examination of more than fifty such larvae 

 failed to show a single case in which a true apical tuft was present. 

 Sections and total preparations reveal the remarkable additional 

 fact that in such larvae, at least in many cases, no apical plate is 

 formed, though the lateral areas of proliferation, referred to 

 above, are present, as shown in Fig. 40, a, a. In a few cases 

 I have found a somewhat vague thickening at the apical pole, 



