208 Heredity and Environment 



are isolated by the partition walls. In some cases, as for example 

 in certain protozoa, the commingling of different kinds of pro- 

 toplasm within a cell may be prevented by the viscosity of por- 

 tions of the protoplasm, or by the formation of intracellular mem- 

 branes, or by a reduction to a minimum of the mitotic movements 

 within the cell by the persistence' of the nuclear membrane dur- 

 ing division. In general the degree of differentiation may be 

 measured by the degree of unlikeness between different cells, and 

 by the completeness with which the protoplasm of different cells 

 is kept from intermingling. 



(3) The Chromosome Theory of Heredity Applied to Embry- 

 onic Differentiation. According to the chromosome theory of 

 heredity the inheritance factors are located in the chromosomes, 

 and the cytological evidence shows that chromosomes always di- 

 vide equally and presumably every cell of an individual contains 

 the same kinds of chromosomes and the same kinds of inheritance 

 factors. How then is it possible to explain embryonic differen- 

 tiation ? How can identical factors give rise to different products 

 in different cells? 



This is evidently due to the fact that while the division of 

 chromosomes is non-differential, that of the cell body is often 

 differential and the same 'chromosomes and genes acting upon 

 different kinds of cytoplasm will produce different results. But 

 differential cell-division is the result of definite movements in the 

 cytoplasm, of definite orientations of spindles and cleavage planes, 

 and ultimately of a definite polarity and symmetry of the cyto- 

 plasm. There is abundant evidence that these cytoplasmic orien- 

 tations are not the immediate results of chromosomal activity and 

 even if some of them may be the remote results of such activity it 

 is logically impossible to place all the differential factors of de- 

 velopment in non-differentiating genes. 



On the other hand if embryonic differentiations are produced 

 by the interaction of chromatin and cytoplasm, and if the chro- 

 matin does not undergo differentiation, it follows that some of the 



