Experimental Studies on Germinal Localization. 37 



apical organ. The only conclusion that can be drawn from this 

 contrast is that the definitive basis of the apical organ is more 

 closely localized in Dentalium than in Patella, being concentrated 

 in a single cell. 



The above results prove that the determination of the develop- 

 ment of the apical organ takes place at some period between 

 the first and the third cleavages. Further experiments fix the 

 period of determination still more nearly. If the egg be allowed 

 to advance as far as the second cleavage and the polar lobe formed 

 at that time be removed, the egg continues to segment in a manner 

 indistinguishable from that of an egg from which the lobe has 

 been removed at the time of the first cleavage. From such eggs 

 arise larva agreeing exactly with those arising after removal of 

 the first polar lobe in every respect save one, namely, that the 

 apical organ is typically present, though this is not invariably 

 the case. (Fig. 36.) Sections clearly show that the apical 

 tuft is borne upon a very definite apical plate, in striking con- 

 trast to the larvae arising after removal of the first polar lobe. 

 It is thus possible to produce at will larvae which lack the 

 post - trochal region and either possess or lack the apical or- 

 gan; and the determination of the apical organ is thus proved 

 to be effected during the short period between the first and sec- 

 ond cleavages. Complete corroboration is given by removal of 

 the second polar lobe from the isolated CD ^ during its first 

 division. The resulting larva resembles that arising from the 

 AB half in having no post - trochal region, but possesses an apical 

 organ as well developed as though the polar lobe had not been 

 removed. (Fig. 37.) 



The experiments just described prove, first, that the correla- 

 tion between post-trochal region and apical organ is due to their 

 common determination by the first polar lobe. The second polar 

 lobe, though apparently precisely similar to the first, has no longer 

 any influence on the apical organ, though it still determines the 

 development of the post-trochal region. It seems impossible to 

 explain these facts, save under the assumption that the first polar 

 lobe contains specific stuffs that are in some manner essential to 

 the formation of both structures, and that during the period 



