REPRODUCTIVE ORGANS. 2l 
_ In a general way, a skeleton performs three functions. 
Firstly, it gives a general firmness or consistence to the whole 
body which from its protoplasmic nature would otherwise 
be mobile. Secondly, it protects the body from enemies, 
physical or organic, and, thirdly, it provides a mechanical 
system of levers through which the muscles can operate. 
Reproductive System.—The vital phenomenon of 
GRowTH does not become concentrated in one special sys- 
tem of organs, though there are striking cases of differential 
growth in many animals. In the case of REPRODUCTION it 
is different, and a special reproductive system appears very 
early in the animal kingdom. The subject of reproduction 
is dealt with under Embryology, but we may note here that 
reproductive organs usually have :— 
1. The primary gonad, producing the germ-cells. The 
male organ is called the ¢es¢is, the female the ovary. 
2. Ducts leading to the exterior. The male duct is 
called the vas deferens, the female the oviduct. 
Further differentiations ensue as development becomes 
more complex. Firstly, the eggs are supplied with yolk 
and yolk-glands are often required. Secondly, the eggs 
require protecting shells or capsules produced by she//-glands. 
Thirdly, these additions require internal fertilisation, within 
the oviduct, before the shell is added. ‘This means copula- 
tion and a copulatory organ in the male, whilst there may 
be a receptacle for the semen (veceptaculum seminis) in the 
female. Lastly, the eggs and young may be retained for 
some time within the oviduct of the female, in which case 
the portion of the oviduct adapted for this purpose is the 
uterus. Various accessory glands may become superadded. 
We may tabulate the reproductive organs as follows :— 
Male.....1. Production of spermatozoa......... Testis. 
2. Transportation to exterior ......... Vas deferens. 
3. Introduction of same into female, Penis. 
Female.,.1. Production of eggs .........cceeeeeee Ovary. 
2. Transportation of eggs to exterior, Oviduct. (glands. 
3. Production of yolk and shell...... Yolk-glands and shell- 
4. Reception of sperms ................ Receptaculum seminis. 
5. Retention of egg and embryo...... Uterus. 
