324 INTRODUCTION TO CYTOLOGY 



Cytological studies have therefore centered mainly about the general 

 organization of the egg (chiefly that of animals) as related to the character 

 of the organism arising from it (the problem of development), and about 

 the roles played by the various cell organs of the gamete in the transmis- 

 sion of heritable characteristics from one generation to the next (the 

 problem of heredity). The character of the principal modern theory of 

 heredity to which these studies have led is due in no small measure to the 

 influence of a number of earlier hypotheses, such as those of Darwin and 

 deVries, and especially that of Weismann. These hypotheses will be 

 reviewed in Chapter XVIII, where their relation to the modern cytolog- 

 ical interpretation of heredity, set forth in this and the following three 

 chapters, will be discussed. 



It is obvious that an account of the physical basis of heredity would 

 require for completeness not only a description of the structural changes 

 by which visible materials are transmitted and distributed during game- 

 togenesis, fertilization, and development; but also a review of many phy- 

 siological processes which accompany these changes, and through which 

 many characters are brought to expression. In these chapters attention 

 will be limited largely to the structural aspects of the problem. 

 Among the physiological changes those occurring at the time of fertiliza- 

 tion are best known, and have already been discussed in Chapter XII. 



The R6le of the Nucleus. — It was Ernst Haeckel (1866) who first 

 advanced the hypothesis that "the nucleus of the cell is the principal 

 organ of inheritance." Cytological evidence in support of this view, 

 announced by Haeckel as a speculation, was brought forward by O. 

 Hertwig (1875 etc.), Strasburger (1878, 1884), van Beneden (1883 etc.), 

 and a number of other investigators, who described the behavior of the 

 nucleus in the various stages of the life cycle, particularly in somatic 

 cell-division, maturation, and fertilization. Two of these workers, O. 

 Hertwig and Strasburger, who had discovered the fusion of the gamete 

 nuclei at the time of fertilization in animals and plants respectively, 

 definitely announced the theory, now supported by a considerable body 

 of observational and experimental evidence, that the nucleus is the 

 "vehicle of heredity." They held that hereditary transmission is 

 through the nuclei of the gametes, and that the chromatin is the special 

 inheritance material, or "idioplasm," about which there had been so 

 much speculation. This view was at once widely adopted by biologists. 



The efforts of many cytologists were now directed toward the further 

 elucidation and verification of this nuclear hypothesis of heredity, and 

 many observations and experiments apparently demonstrated its essen- 

 tial correctness. It was noted that, so far as could be discerned, the 

 spermatozoon in many cases brings nothing but nuclear material into 

 the egg, so that hereditary transmission from the male parent must be 

 through the nucleus alone. A similar condition was later reported in 



