188 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



these lowly forms, therefore, rejuvenation depends 

 on the stimulating properties of cell substances of 

 somewhat different strains. 



The sex in which impregnation occurs is, apparently, 

 by necessity of economy and convenience, the sex 

 on which devolves the task of nourishing the 

 young organism although it is quite within the 

 limits of possibility that an embryo may attach 

 itself and grow within suitably prepared male struc- 

 tures. In nature there are, in fact, a few examples 

 though only among the somewhat lowly forms 

 of male gallantry in at least sheltering the embryo. 

 For example, the male pipefish and sea spiders are 

 provided with blind pouches in which they house 

 the eggs during development. But, in general, the 

 duty of nourishing the embryo involves noteworthy 

 personal sacrifices and discomforts on the part of 

 the female parent, often including, for the human 

 mother, a limitation of muscular and intellectual 

 activities. In the case of the human species, more- 

 over, there must be reckoned also the relatively diffi- 

 cult and painful process of delivery. This mechanical 

 difficulty in childbirth is due to the large size of the 

 child's head, which is merely another way of saying 

 that it is the inevitable result of the long period of 

 growth and protection of the noblest part of the 

 physical mechanism, the cerebrum. 



The period of early adult life is, in general, the 

 time when persons are physically best fitted for the 

 responsibilities of parenthood. Yet it may be said 



