786 DEVELOPMENT 



hybrid animal, germ cells will result that carry white short hair, white long, 

 dark short, and dark long. 



The theory of inheritance through the agency of chromosomes is based 

 upon this type of fact, and the close similarity to the manner of distribution 

 of chromosomes in the maturation of the germ. By the theory the color 

 hybrid is supposed to have a color-producing chromosome from one parent, 

 but from the other parent a corresponding chromosome which lacks this 

 power. In the maturation of the germ cell one of these two corresponding 

 chromatin elements is left alone in each mature cell, otherwise the 

 chromosomes would not have been reduced from a double to a single set. 

 And for any one germ cell the chances are equal whether it receives the color- 

 bearing or the color-lacking chromosome. In either case the cell is "pure" 

 for whichever trait it carries. 



Germ Cells in Relation to the Determination of Sex. A peculiar 

 chromosome or pair of chromosomes has been found in man, and some other 

 vertebrates, as well as in insects, nematode worms and other types of lower 

 animals, which is closely connected with the determination of sex. It is 

 called the heterochromosome, or X chromosome. In its simplest form this 

 chromosome is single in the male, but paired in the female. In synapsis in 

 the male cells it remains separate, because of the lack of a mate. As a result, 

 before the first maturation division it is a triad instead of a tetrad. It can 

 divide only once in the two maturation divisions, and at the other division it 

 passes bodily over to one of the daughter cells, leaving the other cell unsup- 

 plied. Thus there are produced equal numbers of two types of spermatozoa, 

 those with and those without an X element. 



In the corresponding female maturation stages a tetrad is formed, because 

 there are two of these chromosomes present. They behave normally in 

 maturation, and every ripe egg receives one of them. Therefore, after fer- 

 tilization there will be two types of embryos; (i) those receiving two X 

 chromosomes, one of which came from each parent. These develop into 

 females. (2) Those receiving one X chromosome from the mother, and 

 none from the father. These are the males. 



"Sex-linked" Inheritance. This is a peculiar and instructive type of 

 inheritance, which behaves in a way suggesting that the trait may be carried 

 in association with the X chromosome. Its chief distinction is that it is never 

 carried from father to son, the son never receiving an X chromosome from 

 the father. The best human example is hereditary color-bindness, which is 

 a "sex-linked" recessive character, due to some lack in the X chromosome. 

 The presence of one normal color bearing X chromosome is sufficient to 

 establish color vision. 



Changes Following Impregnation. The process of impregnation 

 of the ovum has been observed most accurately in the lower types. 

 The process is as follows: The head of a single spermatozoon joins 

 with an elevation of the yolk substance, the tail remaining motionless and 



