GENERAL EMBRYOLOGY. iSl 



ments, which form the equatorial plate of the nucleus, 

 exactly one half are furnished by the egg-nucleus, the other 

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

 been formed and the contour of the two nuclei has disap- 

 peared, the chromosomes destined for the spindle are com- 

 pletely developed in each of these (Fig. 92). 



Heredity Recent observations have furnished a defi- 

 nite basis for the study 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 Material Basis of Heredity. Since in case of 

 all animals with external fertilization a material connection 

 between parents and offspring can exist only through the 

 sexual cells, consequently these latter must contain the 

 substances which render heredity possible; further, the two 

 hereditary substances, in cases of equal capacity for trans- 

 mission, must be present in the egg and in the sper- 

 matozoon in equal quantity. By this course of reasoning, 

 the chromatic nuclear substance which forms the chromo- 

 somes has come to be regarded as the bearer of heredity ; 

 for we know that the egg contains 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. From this an earlier expressed view, that the 

 nucleus is the bearer of hereditary qualities and determines 

 the specific character of the cell, gains further support (con- 

 sult p. 80). 



