156 INTRODUCTION TO GENERAL PHYSIOLOGY 



cow's milk may not contain sufficient of the anti-scorbutic factor 

 to suffice for the human infant, and may require the giving of fruit 

 juice in addition. The mammary glands increase greatly in size 

 during the time that the young animal is growing in the womb. 

 The stimulus to their growth is provided by a chemical hormone, 

 formed in a peculiar tissue which takes the place of the ovum after 

 it has left the ovary and been fertilised. This structure is called 

 the corpus luteum, from the yellow pigment contained in it (luteus = 

 yellow). The nature of the stimulus which excites secretion of 

 milk when the young animal is born is not yet clear. It has been 

 supposed that an inhibitory hormone is produced either by the 

 growing foetus itself or the placenta, so that when these have left 

 the body of the mother, the mammary gland is freed from the 

 agent which prevented its natural secretory process from being 

 manifested. 



Heredity. It can scarcely fail to arouse astonishment that such 

 minute structures as the ova and spermatozoa contain the potential 

 capacity of developing into organisms similar to the parent organ- 

 isms, even to details. It is obvious that, although the ova of the 

 cat and dog are so much alike, yet there must be represented in 

 them, in some way or other, the characteristics of the particular 

 animal. Various theories, resting on very insufficient evidence, 

 have been put forward, but they need not detain us. 



There are, however, two aspects of the question which require 

 a brief consideration. The first of these concerns the facts of 

 inheritance associated with the name of Mendel, who was abbot of 

 a large monastery near Vienna in the middle of the nineteenth 

 century. He found that certain, inherited characters are subject to 

 two laws. For the sake of illustration, let us take the case of the 

 peculiar "waltzing" mice. If a normal mouse is crossed with a 

 waltzer, the offspring appears to be normal, and it might be thought 

 that the peculiar quality had been lost. But when these offspring 

 are bred together, it is found that the quality reappears in some of 

 their young. So that it was still present, but had been overpowered 

 by the normal quality in the first generation. Hence this quality 

 is called " dominant," while that of waltzing is " recessive." The 

 further remarkable fact is that there is a particular proportion 

 between the number of individuals of the normal kind and that of 

 the waltzers, namely, three of the former to one of the latter. How 

 is this to be accounted for ? Since the original pair may reasonably 

 be regarded as contributing an equal number of the factors in 

 question, we may take it that they are also so present in the total 

 generation of three normal and one waltzer. What happens then 

 is evidently that the factors must be arranged in the following way : 

 one individual contains only dominant factors, another only reces- 



