CLEAVAGE AND DIFFERENTIATION 



an apical organ. It is clear, therefore, that in the egg of Dentaliiim 

 there is a particular portion of the cytoplasm which is precociously 

 chemo-difTerentiated, and essential for the formation of the apical 

 organ and post-trochal region. This portion is contained in the first 

 polar lobe, the contents of which are distributed in a definite and 

 unequal way between the various blastomeres (fig. 53). 



These conclusions are confirmed by experiments (also on Denta- 

 liiim) of a rather diflferent kind, in which the polar lobe is simply 

 cut oiT, without separating the blastomeres. If the polar lobe is cut 

 off at the onset of the first cleavage, the larva (like that from isolated 



a b 



Fig. 54 



Organ-forming substances in Dentalium. a, Normal larva, 24 hours old. b, Larva 

 lacking apical organ and post-trochal region , obtained after removal of the first polar 

 lobe. (From Jenkinson, Experimental Embryology, Oxford, 1909, after Wilson.) 



AB, A,B,ov C blastomeres) lacks the apical organ and post-trochal 

 region. If the polar lobe is cut off at the onset of the second cleavage, 

 the larva possesses the apical organ and lacks the post-trochal region. 

 At the approach of the second cleavage, therefore, the organ-form- 

 ing materials for the post-trochal region become separated from 

 those for the apical organ, for the latter are not included in the polar 

 lobe at its second extrusion ; instead, they presumably migrate into 

 the animal end of the D blastomere where they are in a position to 

 become included in i ^ at the next cleavage (fig. 54). 



Similar occurrences whereby chemo-differentiated substances 

 present in the uncleaved egg are restricted by specialisations of the 



