CLEAVAGE AND DIFFERENTIATION 



to the chromosomes. This theory of differentiation as a 

 genetic restriction is consonant with the nine phenomena 

 enumerated above. 



This conception of the role of the chromosomes in the 

 process of differentiation stands in contrast to the attempt 

 of genetics to explain differentiation. 



The last thirty years have seen as an outgrowth of the 

 well-known Mendelian laws that flourishing branch of 

 biology, genetics, nourished by the knowledge of chromo- 

 some behavior, develop almost to the proportions of a 

 separate science^at least It has a very rich vocabulary of 

 Its own. Mendel's laws are not causal, but statistical con- 

 clusions concerning the regularity with which offspring 

 show the characters of parents. The chromosome theory 

 of Mendelian heredity attempts a causal explanation. 



According to the current conception each chromosome is 

 a string of units, the genes, which are the carriers of hered- 

 ity. The gene theory has been developed by work done on 

 the small fruit-fly, Z)rojo^Az7a, raised in the laboratory under 

 fairly constant conditions of temperature, of food, of per- 

 iodic subjugation to heavy doses of ether, etc. Thus 

 divorced from the rigors of nature these animals imprisoned 

 in large numbers In small glass containers have undergone 

 changes which endure for so many generations that they 

 are without doubt constant. Animals exhibiting these 

 enduring changes are mutants. For each mutation genetics 

 has assigned a place on one or another chromosome; some- 

 times determiners of the same mutation are located on 

 several chromosomes. Among geneticists the gene-theory 

 Is widely accepted. Among biologists, on the other hand, 

 even of those who do not accept the gene theory of the 

 geneticists, the majority agree that the chromosomes are 

 the bearers of heredity. 



According to the geneticists the chromosomes In every 

 cell of the most complex organism are identical with those 

 of the fertilized egg. That Is, as they express It, the chro- 



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