662 THE PHYSIOLOGY OF REPRODUCTION 



relations which exist between the amount of growth and the 

 time of growth. He concludes that there are two or more 

 growth cycles representing autocatalytic processes which make 

 up the total growth of an individual. In Man there are three 

 maxima of rate of growth, representing three phases or growth 

 cycles. One of these is intra-uterine, but it is probable that 

 this is not quite complete at birth. The second growth cycle 

 seems to attain its maximum annual increment at about the 

 fifth year, since the increment in weight at that age, as deduced 

 from an investigation on growth in English boys, considerably 

 exceeds the annual increments for the years immediately follow- 

 ing. A third maximum in yearly increments occurs in males 

 at about the sixteenth year, that is, at about the time of puberty. 

 In the rat, according to Donaldson,^ there are two intra-uterine 

 growth cycles, while there is only a single well-defined extra- 

 uterine cycle. Robertson suggests that the first growth cycle 

 in Mammals represents the course of the autocatalytic synthesis 

 of the nuclear substance, that the third cycle represents the 

 period during which cytoplasmic material is built up, while the 

 second growth cycle is intermediate, representing a time when 

 both synthetic processes go on contemporaneously. 



Growth of the Body before Birth 



Minot ^ has recorded the results of weighing embryo rabbits 

 at different stages of development with a view to determining 

 their rate of growth. The results showed that in the period 

 from the ninth to the fifteenth day the young rabbit adds on an 

 average 704 per cent, to its weight daily, and that in the period 

 from the fifteenth to the twentieth day, the average daily addition 

 is only 212 per cent. It may be assumed, therefore, that in 

 younger embryos (before the ninth day) there is an increase of 

 over a thousand per day. Minot estimates that over 98 per cent, 

 of the original power of growth of the rabbit or the chick has 

 been lost at the time of birth or hatching, and that a similar fact 

 is equally true of Man. " We start out at birth certainly with 



' Donaldson, " A Comparison of the White Rat with Man in respect to the 

 Growth of the Entire Body," Boaa Anniversary Volume, Anthropological 

 Papers, New York, 1906. ^ Minot, loo. cit. 



