268 GENERAL ZOOLOGY 



Somewhat similar eye defects were produced by Little and 

 Bagg on young mice whose mothers were exposed to X rays 

 during pregnancy. The defects in this case were also inherited. 



It is probably significant that the same parts, viz., the eyes, and 

 to a certain extent the brain, were affected by such different 

 harmful agents as alcohol, X rays, and antilens substance. Thus 

 in all cases the head region of the embryo seemed most sensitive 

 to these destructive agencies, which in view of the fact that the 

 work of C. M. Child and others has demonstrated that the head 

 end of an embryo is the region most susceptible to the action of 

 harmful agents, such as potassium cyanide, suggests that the 

 results obtained were due to a general effect on the most sus- 

 ceptible part of the body. Since alcohol, antilens serum, and 

 X rays, under the conditions of the experiments, all seem to 

 produce similar results in mammals, particularly in regard to the 

 eye, the idea of specificity of effect, so far as the antilens serum is 

 concerned, is rather difficult to maintain. The skeleton defects 

 noted especially by Stockard and Papanicolaou may well be 

 secondary results following the primary injury to the region 

 of highest metabolism, viz., the head. 



Germ Track. — The failure of efforts to produce changes in the 

 hereditary material or germ plasm through the somatic cells is in 

 keeping with the idea of a germ track or lineage of germ cells 

 connecting generations of individual animals, more or less distinct 

 from somatic cells. Somatic cells undoubtedly influence the 

 germ cells physiologically but not genetically, so far as known. 

 Weismann regarded the germ track as a device for conserving the 

 hereditary potencies of the chromosomes, but since somatic cells 

 receive the same assortment of chromosomes as the germ cells, 

 he believed that the chromosomes of the somatic cells during 

 cleavage and differentiation underwent a kind of differential treat- 

 ment, which he considered to be the cause or basis of differentia- 

 tion. Something of this sort is indicated in the process of 

 chromatin diminution of Ascaris, but since this is not of general 

 occurrence in other species, it cannot be accepted as the general 

 cause of differentiation. If it is generally true that as a result of 

 cleavage both somatic and germ cells receive the same kind and 

 number of chromosomes present in the fertilized egg, every 

 somatic cell receives the same complement of genes from the 

 egg. Differentiation of somatic cells presumably results from an 



