154 CYTOLOGY chap. 



A. THE EQUALITY OF INHERITANCE FROM MALE AND 



FEMALE PARENTS 



The fact that, on the average, offepring inherit with approximately 

 equal intensity from both parents, whereas the macrogamete as a whole 

 is nearly always enormously larger (often a million or even more than 

 a biUion times larger) than the microgamete, immediately suggests that 

 the hereditary substratum is not the substance of the gametes as a whole, 

 but some special portion of them which is of more approximately equal 

 mass in the two cells. This consideration led Nageli in 1884 to postulate 

 two substances in the gametes, one of which is present in equal amount in 

 the micro- and macro-gamete. This is the bearer of hereditary qualities — 

 the idioplasm. The other has mainly a nutritive function and is present 

 in far greater amount in the egg and is indeed responsible for its larger 

 size. Knowledge of the processes of fertilization naturally led to the 

 idioplasm being identified with the nucleus (independently by O. Hertwig 

 and Strasburger in 1884), since the nuclear substance appears to be the 

 only one that is contributed in approximately equal amounts by the 

 two gametes. Mere study of the anatomy of the gametes therefore at 

 once leads us to suspect the all-importance of the nucleus and the 

 essential passivity of the cytoplasm in the transmission of hereditary 

 qualities. 



It is of course not necessary to assume that equal quantities of living 

 matter produce equally powerful effects, nor that inequal masses cannot 

 produce equal effects. It is certain also that the amount of chromatin 

 in the nucleus cannot always be taken as denoting the amount of 

 idioplasm therein. Nevertheless, in corresponding stages of the Ufe- 

 cycle such as the ripe male and female gametes, one would expect to 

 find approximate mass equality of the idioplasm, at any rate not the 

 enormous disproportion that exists between the cytoplasm of the egg 

 and that of the spermatozoon. 



The statement just made, that the amount of chromatin in the nucleus 

 cannot always be taken as an indication of the quantity of idioplasm is 

 supported by many considerations. Thus the amount of chromatin in 

 the nucleus varies greatly in more or less closely allied forms (Fig. 71). 

 We cannot suppose that the amount of idioplasm in the nucleus of 

 Lepidosiren is much greater than in that of the salamander, and enor- 

 mously greater than in the nucleus of a rabbit. Moreover, the amount 

 of chromatin varies in different tissues of the same organism, or even 

 in different periods of the Hfe-cycle of the same nucleus. Riickert pointed 

 out long ago that the amount of chromatin in the oocyte nucleus in the 

 growth period (Selachians) is very much greater than the combined 

 bulk of the chromosomes on the spindle of the first maturation division. 



