SEXUAL CHARACTERISTICS OF THELIA 617 
It must be assumed that the gametes themselves have under- 
gone an evolution and that many genetic changes have occurred 
to modify and specialize the form of the mature germ cell. The 
multitudinous forms and the intricate apparatus which sper- 
matozoa exhibit lend credence to this idea. Likewise, the ovum 
must have been changed from its original state. One of the first 
processes, one which has now become universal in animal ova, 
must have been the production of cells of unequal size in matu- 
ration. The polar cells represent abortive ova, whereas the one 
functional ovum receives the nutritive supply originally intended 
for four cells of equal size. Thus fewer and larger ova would 
result from this change. In many animals the period of growth 
was prolonged so that larger ova resulted, finally culminating in 
the immense egg cells of birds. Undoubtedly mutations affect- 
ing the storage power of the somatic cells as well as the germ 
cells have aided in this specialization and caused the female to 
diverge in its physiological constitution from the male. This 
difference is still demonstrable between the male and female 
somas of mammals. Although the ovum no longer stores yolk, 
still the body tissues must provide food for the growing embryo. 
An economy is perhaps hereby effected in that fertilization must 
first be insured before prolonged storage of food materials can 
take place. 
With the specialization of the gametes changes in the acces- 
sory tissues of the gonads, in their ducts and glands, must have 
becomes established. These tissues minister to the needs of the 
gametes and serve to carry them to the exterior. In land ani- 
mals and also in some aquatic animals copulation and internal 
fertilization made the development of external genital apparatus 
essential and the establishment of instinct of sex necessary. 
Last of all, we are to look upon the extragenital sexual char- 
acteristics, including all forms of ornamentation, as coming into 
existence. 
According to the views expressed above, sex has evolved 
through a series of genetic changes accompanying the evolution 
of the various groups of the animal kingdom. It is not assumed 
to have sprung up independently whenever a difference in the 
