414 INTRODUCTION TO CYTOLOGY 



constitution, i.e., the physical and chemical properties of the germ-plasm 

 rather than the substance itself. 



Conclusion. In the foregoing pages we have touched upon some of 

 the most important biological problems toward the solution of which 

 cytology must make her further contributions. With regard to individual 

 development it must be determined on the one hand to what extent the 

 course of ontogenesis is dependent upon the operation of an internal cell 

 mechanism and how this mechanism brings about its results, and on the 

 other hand how far it is controlled by external environmental agencies: 

 a way must be found between the "Scylla of preformation and the 

 Charybdis of epigenesis" (Conklin 1913). Furthermore, the manner 

 and the causes of the progressive modification of the hereditary mechan- 

 ism must be better known in order that evolutionary advance 

 may be accounted for. With respect to both development and heredity 

 the roles of the two individualities, the cell and the organism as a whole, 

 must be more fully ascertained and correlated. 



It is obvious that no adequate solution of any of these problems can 

 be reached until the physico-chemical constitution of protoplasm, 

 especially that of the idioplasm or inheritance material, is more 

 clearly disclosed. Only further research can show whether we shall 

 continue to regard the idioplasm or chromatin as a heterogeneous 

 system of discrete molecules or molecular complexes (factors or genes) 

 with a definite spatial arrangement, as is supposed on our current 

 Mendelian theories, or shall come to look upon it as a single enormously 

 complex chemical substance in which varying side-chains or other portions 

 of the molecule are responsible for the variety of results observed. It is 

 at any rate a striking fact that "in the Mendelian phenomenon we see a 

 synthesis, splitting apart, and recombination of determinative factors 

 that is singularly like that of chemical elements or radicles" (Wilson 1909, 

 p. 108); and nothing appears more clearly evident than the truth of 

 Wilson's assertion that ". . , in the union of cytology and biochemistry 

 lies our greatest hope of future advance." 



Bibliography 14 



Heredity; Sex 



ADAMI, J. G. 1918. Medical Contributions to the Study of Evolution. London. 

 ALLEN, C. E. 1917. A chromosome difference correlated with sex differences in 



Sphcerocarpos. Science 46: 466-467. 

 1919. The basis of sex inheritance in Sphcerocarpos. Proc. Am. Phil. Soc. 58: 



289-316. figs. 28. 



ALTENBTJRG, E. 1916. Linkage in Primula sinensis. Genetics 1 : 354-366. 

 AMMA, K. 1911. Ueber Differenzierung der Keimbahnzellen bei den Kopepoden. 



Arch. Zellf. 6: 497-576. figs. 25. pis. 27-30. 

 BABCOCK, E. B. and CLAUSEN, R. E. 1918. Genetics in Relation to Agriculture. 



New York and London. (Bibliography.) 



