MENDEL'S LAWS OF HEREDITY 



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the male pronucleus, or sperm-nucleus. The tail of the spermatozoid 

 disappears. 



Before the penetration of the spermatozoid, the ovum has sent off 

 the polar bodies, and the number of chromosomes has been reduced 

 one-half. What remains of the germinal vesicle, after the separation of 

 the polar bodies, passes to the centre of the ovum and forms the female 

 pronucleus, or egg-nucleus. The male pronucleus now unites with the 

 female pronucleus to form the cleavage-nucleus, which contains the same 

 number of chromosomes as the ordinary tissue-cells, the number in each 

 pronucleus having been reduced one-half. The centrosome of the ovum 

 has disappeared and the centrosome of the spermatozoid, which has been 

 contained in the middle piece, divides into two, one passing to either 

 side of the cleavage-nucleus. Under this view, division of the ovum can 

 not take place without the introduction of a male centrosome. It is 

 thought that hereditary qualities are transmitted in the chromosomes. 

 The cleavage-nucleus thus contains chromosomes, one-half of which are 

 from the ovum and one-half from the spermatozoid. 



Mendel's Laws of Heredity. About the year 1860, Gregor Mendel, 

 an Austrian monk and botanist, in studying hybrid forms of certain 

 plants, observed peculiarities transmitted from parent-stocks which 

 he formulated into what is now known to biologists as Mendel's law, 

 or, more properly, Mendel's laws. He found that hybrids nearly 

 always presented the characters of one parent-stock only ; especially 

 when each parent-stock was pure. The parent whose peculiarities 

 were thus represented in the offspring presented characters that he 

 called dominant ; but the characters of the other parent, though 

 undeveloped in the immediate offspring, remained dormant, possibly to 

 reappear later. These latter characters he called recessive. 



This result of cross-breeding, however, was not constant. In some 

 offspring, the dominant character of one parent was not only repeated 

 but became exaggerated ; sometimes the hybrid showed individual 

 characters, not possessed by either parent and differing from both, 

 possibly derived from an anterior ancestral condition ; but the cross 

 always presented the same characters in case the germ-cells from the 

 two parent-stocks were pure. The principle thus illustrated in plants 

 was shown to exist in crossings of animals, as white and gray mice. In 

 crossing pigmented with albino animals, the pigment character was 

 always dominant and the albinism, recessive. 



Without discussing fully the varied and frequent changes in char- 

 acter produced by crossing hybrids with hybrids and hybrids with either 

 one of the pure parent-stocks, the following may be taken as the prin- 

 ciples underlying Mendel's laws : 



