154 INTRODUCTION TO GENERAL PHYSIOLOGY 



are laid before fertilisation, as in the frog or fish, the male is 

 required merely to deposit a liquid containing spermatozoa over 

 the mass of eggs. In birds, although the eggs are laid and develop- 

 ment proceeds outside the parent organism, the presence of the 

 hard shell necessitates the introduction of spermatozoa into the 

 duct of the ovary, so that they may enter the ovum before the 

 outer layers are deposited on it. For this purpose, the male brings 

 the orifice of the tube down which the secretion of the testis is 

 poured into contact with the orifice of the oviduct and ejects the 

 spermatozoa into the oviduct. A similar process is, of course, 

 necessary in the mammalia, and in special cases in the lower 

 organisms, where the fertilised ovum remains within the mother's 

 body and completes its development up to an advanced stage 

 therein. Here the male is provided with a special organ, the flews, 

 for the purpose of more effective introduction of the spermatozoa 

 into the oviduct of the female. The act of doing this is associated 

 with feelings of pleasure in both sexes, as is obviously necessary 

 to ensure the continued existence of the race by the production 

 of offspring. It may be noted that a process of the kind is often 

 present although the fertilised eggs are afterwards laid and develop 

 outside the body of the female, as in many invertebrates. 



The large size of the female gamete, or egg, as compared with 

 the male gamete, especially striking in the bird and reptile, is 

 miinly due to the fact that it is provided with a store of food 

 material for the growth of the young animal, but is also used by 

 ourselves for food. In the mammal, the food material is supplied 

 by diffusion from the blood of the mother to that of the growing 

 embryo. An organ, in which ramifying networks of blood-vessels 

 from both sources lie side by side, known as the placenta, is present 

 for this purpose. 



A word may be said here in regard to flowers. It is by no 

 means always realised that their beauty is, directly or indirectly, 

 connected with the process of sexual reproduction (E., p. 227). This 

 fact that they are, as has been said, "naked and unashamed," and 

 the absence of any suggestion of unseemliness should give us 

 matter for thought. Although the male and female gametes, 

 pollen grains, and ovules, are usually parts of the same flower, a 

 variety of devices exist in order to favour cross-fertilisation. The 

 pollen grains are not motile, but in some cases, as the grasses, they 

 are produced in enormous quantity, and, being very light, are blown 

 about by the wind. In other cases, the brilliant colouring, the 

 scent, and so on, serve to attract insects of various kinds to suck 

 the sugary solution, nectar, from the depths of the flower. In 

 doing this, they brush against the stamens and carry away pollen 

 from them. A visit to another flower deposits part of the pollen 



