SYMBIOSIS 45 



this must have been generally the case in evolution. We can 

 also see how great a part must have been played by every form 

 of protracted nutritional gestation in pre-determining new 

 organs and new structures, for, as we have recognised, nutri- 

 tion determines the degree of symbiosis and, hence, also the 

 possibilities of metabolic response to all new and important 

 factors affecting a species. 



In the female mammal a considerable amount of repro- 

 ductive labour and connected metabolic activity is deputed to 

 an important gland the corpus luteum which, by means of 

 chemical stimulation, controls (so far as we yet know) the 

 uterine changes necessary for the attachment of the embryo in 

 the early stages of pregnancy, and also incites the formative 

 growth of the mammary glands. 



As soon as the corpus luteum is formed, the mammary 

 gland begins to grow r . This growth is noticeably increased 

 after the body is fully formed, a stage characterised by the 

 presence of plentiful granules in the lutein cells. The gland 

 secretes a substance of lipoid nature. It is present in a well- 

 developed condition in the three sub-classes of the Mammalia, 

 and it plays a most important part in the mechanism of repro- 

 duction. "We have seen the connection between " love-foods " 

 (with the implied degrees of symbiosis) and the mammary 

 gland. Can we now deny in view more particularly of the 

 bio-chemical importance of the corpus luteum that biological 

 symbiosis, i.e., an adequate symbiogenesis, must have been the 

 pre-requisite of its origin and evolution? 



The protracted ingestion of "love-foods" and of vita- 

 mines under genuinely symbiotic relations increasing in 

 adequacy and in abundance with every increasing degree of 

 symbiosis has, I believe, in course of time tended to build up 

 the corpus luteum as a special organ to grapple with the more 

 complicated problems of Bio-Chemistry offered by the higher 

 forms. Its evolution has been no different from that of the 

 mammary gland. It coincided with the perfection of 

 symbiosis. In the last analysis, of course, it may be 



