GROWTH AND REPRODUCTION 155 



on the stigma, whence the pollen tube grows down through the 

 tissue and fertilises the ovule. 



There are many familiar differences, of no apparently serious 

 importance, between the two sexes in the greater number of 

 animals. These are the ''secondary sexual characters" and may 

 be illustrated by the hairs on the face in man, the mane of the lion, 

 the horns in some breeds of sheep, the crest on the newt, and so on. 

 It is found that they are due to the internal secretion of the sex- 

 glands, and indeed to that of a particular kind of cell, found both 

 in the ovary and in the testis, although, of course, the nature of 

 the secretion is not the same. These are known as the interstitial 

 cells. They are quite independent of the germ plasm from which 

 the ova and spermatozoa arise. The chemical substances respon- 

 sible for the effects seem to be sometimes inhibitory, preventing 

 the growth of hair on the face in women, for example ; sometimes 

 excitatory, as in the Herdwick ram, where removal of the testis 

 in the young animal stops any further growth of the horns. 



Attempts to assign the difference between the properties of 

 maleness and femaleness to general physiological differences in 

 metabolism cannot be said to have met with great success. Thus 

 it has been suggested that the female is more prone to the synthetic 

 or "anabolic" changes, the male to the "catabolic," or breaking 

 down processes, supposed to be the bases, respectively, of inhibitory 

 and excitatory phenomena (see P., pp. 421-423). But the view 

 that food material is made into a complex protoplasmic molecule, 

 before being oxidised to afford energy, has been practically given 

 up as our knowledge of cell processes has grown. Indeed, the 

 conception of the universal occurrence of anabolic and catabolic 

 stages as parts of the same chemical reactions does not seem to 

 hold. In its application to the two sexes, it is pointed out that the 

 male is the more active and enterprising, the female slower and 

 more conservative. But the reader will be able to call to mind 

 many cases to the contrary, and the distinction, like many other 

 supposed sexual ones, is probably not of this nature at all, but 

 merely incidental. 



Although the chick, when it is hatched from the egg, is able 

 1 3 pick up its own food, it is a familiar fact that the young of 

 mammals are comparatively helpless for some time after birth, so 

 that they depend on being fed with milk, secreted by special glands 

 possessed by the mother. These mammary glands, as they are 

 called, have been developed in the course of evolution from glands 

 in the skin. Milk itself contains all the constituents required by 

 a complete diet, such as we learned in our second chapter a 

 sugar, lactose, fat in small globules, protein of two kinds, salts, and 

 the necessary accessory factors. It should be noted, however, that 



