CLEAVAGE AND DIFFERENTIATION II7 



its three sisters is doubtless to be explained by the presence in it 

 alone of such specific organ-forming stuffs. 



Summing up the evidence, we may say that the animal pole- 

 plasm, and its presumable homologue, the slightly thickened cap of 

 cytoplasm at the animal pole of the egg of De?italmm, are in some 

 way responsible for normal cleavage, though this has only been 

 demonstrated for Clepsine (see above). On the other hand, organ- 

 forming substances for apical organ (where present), and mesodermal 

 and ectodermal germ-bands, appear to be located in the vegetative 

 pole-plasm or polar lobe. This has been demonstrated for the apical 

 organ in Dentalium, for the mesodermal germ-bands in all forms 

 investigated, and for the ectodermal germ-bands in Dentaliiim. The 

 migration in the direction of the animal pole of the material for the 

 apical organ occurs before the second cleavage {Dentaliiim), for the 

 ectodermal and mesodermal germ-bands only later, in some cases 

 (Annelida) after a union of the two pole-plasms within the D 

 macromere, to be segregated at the fourth and sixth cleavages 

 respectively. 



In general we may say that determinate spiral cleavage provides 

 an effective method of distributing precociously differentiated 

 substances to particular regions of the embryo, and that special 

 advantage of this has been taken by the Annelids and Molluscs, 

 though in varying degrees by different forms. 



§5 

 The conditions found in Bero'e and Dentalium introduce us to 

 another principle of considerable importance. In Beroe, the forma- 

 tion by chemo-differentiation of the green ectoderm-producing 

 substance and the uncoloured endoderm-producing material is 

 effected prior to fertilisation, but the localisation of these substances 

 in their definitive positions is only brought about during cleavage. 

 In regard to the tgg, these substances are preformed, but not 

 prelocalised. 



The same is true, though the details are even more elaborate, 

 concerning the distribution of the materials contained in the pole- 

 plasms and polar lobes of Mollusca and Annelida. 



When the distinction between mosaic- and regulation-eggs was 

 regarded as fundamental, this distinction between the preformation 



