126 DOMESTICATED ANIMALS AND PLANTS 



female on another, following the plan of the higher animals, 

 in which the two sexes are always identified with separate 

 individuals. See also Fig. 19, which shows that the tassel is a 

 modified ear with the female flowers normally undeveloped. 



In the higher animals the ova are produced periodically and 

 fertilization is variously effected. In fishes, for example, the 

 eggs are fertilized by the male after having been deposited by the 

 female. In frogs the eggs are fertilized during their deposition. 

 In birds the eggs are " laid " as fast as they mature, but unless 

 they have been fertilized by the spermatozoa of the male previous 

 to being laid, they will not " hatch," just as the unfertilized ovules 

 of the corn fail to develop, leaving the cob bare of kernels. 



In mammals the ova ripen periodically like the eggs of the 

 bird, with this difference, that if fertilized before escaping from 

 the body, they are not discharged at all, but are retained in the 

 uterus of the mother during embryonic development and are 

 carried there until birth. The ova of mammals, unlike those 

 of birds, are not supplied with sufficient nutriment to last 

 through their comparatively long period of development, and 

 this prenatal food is supplied directly through the blood of 

 the mother. 



The student hardly needs to be reminded that this blending 

 of nuclei takes place and development follows only when the 

 nuclei are not too dissimilar. For example, wheat would not be 

 fertilized by pollen of corn, but it has been fertilized by that 

 of rye. This mixing of very different races is known as hybridi- 

 zation. The most frequent case of hybrids among animals is 

 the common mule, but a hybrid has been made between the 

 lion and the' tiger and very frequently among plants, as between 

 the raspberry and the blackberry. We pass now to a more care- 

 ful consideration of what is involved in transmission. 



The material transmitted. All that is " handed down" from 

 parent to offspring is, therefore, the minute bit of matter con- 

 tained in the two nuclei and the small amount of surrounding 

 protoplasm, — microscopic in almost all cases. Of course a single 



