574 THE HUMAN BODY 



of the ovum, at this stage called the female pronucleus, and the two 

 fuse into a single fertilization or segmentation nucleus. This process 

 restores to the ovum the typical number of chromosomes. 



In addition to the chromatin material the spermatozoan also 

 brings with it the stimulus to cell division, so that immediately 

 after the formation of the fertilization nucleus segmentation begins. 

 In the first and subsequent divisions, which, as stated earlier, are 

 by the process of mitosis, the chromatin is so distributed that each 

 daughter cell receives equal amounts from ovum and sperm. 



Heredity. The relative influence of the two parents upon the 

 characteristics of the offspring has been studied and speculated 

 upon for ages. With the discovery of the chromosomes it has 

 become evident that to them we must look largely, if not wholly, 

 for the agency of hereditary transmission. So far as paternal 

 characters impress themselves they must do so through the 

 chromosomes since the sperm contributes virtually nothing else. 

 To the Austrian monk Mendel and the Dutch botanist DeVries 

 we owe the conception of the machinery of heredity which has 

 clarified our ideas on the subject more than all the previous work 

 has done. 



According to this conception the chromosomes are to be looked 

 upon as made up of groups of determiners of hereditary traits. If 

 in the union of maternal and paternal chromosomes all the factors 

 are harmonious the offspring will be a perfect blend of the parents. 

 This condition is not realized, however, unless the parents are 

 alike in practically all respects. Thus if one is light haired and 

 the other dark, or if one has blue eyes and the other brown the 

 chromosomes which bear these traits are in conflict. Mendel 

 found that under these circumstances usually one of the conflicting 

 traits appears in the offspring and the other is suppressed. The 

 one which appears is called dominant, the other recessive. More 

 rarely there is a blending of the characters, as seen in the inter- 

 mediate skin coloration in mulattos. Even though the recessive 

 traits are not apparent in the presence of dominant conflicting 

 characters the chromosomes which determine them persist un- 

 changed, and will be found in the germ-cells. An individual whose 

 germ plasm contains such conflicting chromosomes is known as a 

 hybrid. Experiment has shown that during the development of 

 the germ plasm of hybrids there is a separation of conflicting char- 



