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differentiating genes on different parts of the cyto- 

 plasm; but there is no mechanism by which embryonic 

 differentiations could come from the action of non- 

 differentiating genes on a homogeneous cytoplasm. 

 The genes or Mendelian factors are undoubtedly 

 located in the chromosomes, and they are sometimes 

 regarded as the only differential factors of develop- 

 ment, but if this were true these genes would of 

 necessity have to undergo differential division and 

 distribution to the cleavage cells, as Weismann 

 maintained. Since this is not true it must be that 

 some of the differential factors lie outside of the 

 mcleus, and if they are inherited, as most of these 

 ;arly differentiations are, they must lie in the 

 ;ytoplasm. 



Rabl C'06) conceived of differentiation 

 as brought about by a continuous inter- 

 action between nucleus and cytoplasm, — 

 the primary germinal localizations (organ 

 forming stuffs) of the cytoplasm furnishing 

 the starting point. According to him 

 both nucleus and cytoplasm are progres- 

 sively changed during the process of 

 differentiation. 



Wilson, on the other hand, throws the 

 entire responsibility upon the nucleus. 

 "Heredity," he says, "is effected by the 

 transmission of a nuclear preformation, 

 which in the course of development finds 

 expression in a process of cytoplasmic 

 epigenesis." 



Loeb C'16) viewed the cytoplasm of the 

 egg as " 'the embryo in the rough,' carry- 

 ing the genus — or even the species — 

 heredity, while Mendelian heredity adds 

 only the finer details to the rough block." 

 Jenkinson Qij) adopted a similar view, 

 holding that the larger characters are 

 transmitted by the cytoplasm, the smaller 

 by the nucleus. Conklin is more specific. 

 "We are vertebrates because our mothers 

 were vertebrates and produced eggs of the 

 vertebrate pattern; but the color of our 

 skin and hair and eyes, our sex, stature 

 and mental peculiarities were determined 

 by the sperm as well as by the egg from 

 which we came." But as Wilson further 

 says, "These statements are rhetorically 

 effective, but will not stand the test of 



critical analysis .... the cytoplasmic 

 organization of the egg is itself the product 

 of an antecedent -process of epigenetic develop- 

 ment in the course of which, as we have 

 every reason to believe, the chromosomes 

 have played their part, — thus the chromo- 

 somes are as much concerned in the 

 determination of the so-called 'preformed' 

 or cytoplasmic characters as in any 

 others." 



Professor Morgan's position is very 

 similar to that of Wilson. Starting with 

 the assumption of a chromosomal archi- 

 tecture as outlined above, he brings out the 

 following points : (1) There is no relation 

 between the arrangement of the genes and 

 the arrangement of the parts differentiated 

 from the egg. (2.) The genes remain 

 intact throughout development. There 

 is no evidence for assuming that they are 

 altered by successive cytoplasmic differ- 

 entiations. (3) There is no basis for 

 assuming that the genes become active in 

 a particular order nor can it be stated 

 whether all the genes are active continu- 

 ally or whether some are more active than 

 others at certain periods. 



In short, about all that Professor 

 Morgan feels sure of is that "the genes 

 may change the cytoplasm and the cyto- 

 plasm then acts in a specific way" (p. 8). 



How the chromosomes, or rather the genes that 

 are their essential constituents alter the cytoplasm 

 of the cells is unknown. There is at least no evidence 

 opposed to the view that they do this by chemical 

 processes. It is impossible to suppose that the genes 

 themselves could be thrown off and could pass 



through the nuclear membrane into the cell 



It is not so difficult to imagine that through their 

 activities chemical substances are produced that find 

 their way into the cytoplasm either at the time when 

 the nuclear wall is dissolved and nuclear sap set free, 

 or possibly by diffusion through the nuclear walls. 



Cp. 198). 



The genes may be influential in the forma- 

 tion of catalytic materials but are probably 

 not themselves enzymes. 

 The evidence from matroclinous in- 



QDAR. REV. BIOL., VOL. Ill, NO. 3 



