150 GENERAL PRINCIPLES OF ZOOLOGY. 



the other by the sperm-nucleus. For, even before the spindle has 

 been formed and the contour of the two nuclei has disappeared, 

 the chromosomes destined for the spindle are completely developed 

 in exactly the same number in each of these (fig. 95). 



FIG. 95. Fertilization of Ascaris megalocephala . (After Boveri.) A, the ends (centro- 

 somes)of the spindle formed; B, the spindle completed; sp, sperm-nucleus with 

 its chromosomes; e/, egg-nucleus; p, polar bodies. 



Heredity. Recent observations have furnished a certain basis 

 for the doctrine of heredity. By heredity we understand the 

 transmission of parental characteristics to the offspring. This 

 transmission, on the whole, takes place with equal energy from 

 the father's and from the mother's side; if we take the average of 

 numerous cases, the result is that the child's peculiarities hold the 

 mean between the peculiarities of father and mother; or, in other 

 words, male and female individuals in the average have an equal 

 power of transmitting characteristics. 



The Physical Basis of Heredity. Since in case of all animals 

 with external fertilization a material connexion between parents 

 and offspring can exist only through the sexual cells, these latter 

 must contain the substances which render heredity possible; 

 further, the two hereditary substances, in cases of equal capacity for 

 transmission, must be present in the egg and in the spermatozoon 

 in equal quantity. By this course of reasoning, the chromatic 

 nuclear substance which forms the chromosomes has come to be 

 regarded as the bearer of heredity; for we know that the egg con- 

 tains a great quantity of cytoplasm, but the spermatozoon only the 

 slightest trace of it; that, on the other hand, egg- nucleus and 

 sperm-nucleus furnish equivalent substances, and particularly the 

 same quantity of chromosomes, to the cleavage spindles ; hence only 

 the chromatin can be regarded as the hereditary substance (idio- 

 plasm). This supports the view expressed before (p. 67) that the 

 nucleus is the bearer of hereditary qualities and determines the 

 character of the cell. 



