CLEAVAGE AND DIFFERENTIATION 



become pluripotent. The condition of the spermatozoon 

 is discussed later. 



Any theory offered to account for the differentiation that 

 takes place during cleavage should be consistent for both 

 the cleavage-period and the succeeding periods of develop- 

 ment during which differentiation occurs, since in essen- 

 tials, every period of differentiation is alike. 



Certain phenomena occurring in the organism as egg, as 

 cleaving mass, as embryo and as adult are based on some 

 differentiation. A theory of differentiation during cleav- 

 age should hold for these phenomena. They are: (i) 

 polyembryony and experimental twinning; (2) merogony; 

 (3) the development of diploid fragments; (4) the develop- 

 ment of isolated blastomeres; (5) haploid parthenogenesis; 

 (6) experimental and natural polyploidy; (7) asexual repro- 

 duction by budding and fragmentation, and alteration of 

 generations; (8) regeneration of lost parts; and (9) the origin 

 of tumors. I shall now endeavor to show that these phe- 

 nomena are better explained on a theory of differentia- 

 tion as genetic restriction than on one that postulates 

 segregation.^ 



I. The capacity of some eggs, as those of certain insects, 

 normally to produce many embryos from one egg as well as 

 the possibility of producing twins experimentally from 

 normally mono-embryonic eggs including those of the 

 determinate type, as the eggs of Nereis, Chaetopterus, 

 Tubifex, etc., shows that eggs have more latent potency 

 than that required for producing one animal. A theory 

 postulating a restriction of potencies seems to meet these 

 facts better than one suggesting that materials for the 

 embryo are segregated. 



1 Cf. Perez i^igiz) who lists much the same experimental evidence 

 against the mosaic-theory (aji earlier form of the segregation-theory) 

 of development, which he justly considers a modern version of the 

 preforynation doctrine. 



