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 Heine 
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 
