432 ON INCREASE IN SIZE [pt. iii 



long period between 14 days and birth. The cause of this difference 

 in results is due to the fallacy in the method of analysis employed by 

 Minot". This is only true, subject to confirmation of the fact that 

 the foetal log. weight/age graph gives a straight line over definite 

 periods, and this is just what is not certain. Decision on the matter 

 cannot yet be made. 



Another interesting point which emerges from Table 55 is the 

 long embryonic stage in the guinea-pig. The chick hatches when 

 its k is about 0-21 and the rat is born when its k is even higher— 0-53, 

 but the guinea-pig stays inside the uterus until its instantaneous 

 percentage growth-rate has dropped to 0-05. Brody succeeded, indeed, 

 in raising guinea-pigs by feeding them on hay and grain immediately 

 after birth, so that they tasted neither colostrum nor milk. 



It is interesting, again, to note that there is only one break in the 

 instantaneous growth-rate of carbon dioxide production, whereas 

 there are at least two in the instantaneous growth-rate of wet weight. 

 This must mean either that the respiratory function develops at a rate 

 quite independent of the growth in mass, or that the weight of the 

 body cannot be taken as an index of the growth of the metabolising 

 tissues. This point will be referred to again, for it is of much importance 

 in chemical embryology. On the other hand, the respiration k does 

 show a break about the 17th or i8th day, which is duplicated in 

 the wet weight k, or, at any rate, in the log. weight curve con- 

 structed from Lamson & Edmond's data — for it is not so apparent 

 in those of Hasselbalch and of Murray. This may be associated, as we 

 have seen already is Brody's suggestion, with the change in form of 

 respiration occurring then (chorio-allantoic to pulmonary). There is 

 no doubt that some obscure events are associated with this late 

 stage in the chick, e.g. the mortality peak of Payne, which can be 

 greatly intensified if a certain lethal gene is present, and the sudden 

 immunity to implanted rat sarcomata (Murphy), which the chick 

 then acquires. Brody suggests that the chick embryo passes at this 

 stage through a "metamorphosis" similar to those hidden ones which 

 exist, according to Davenport, in the development of man. 



The extremely small values of k for the embryonic period of man 

 are worth attention. The human embryo grows a great deal more 

 slowly than any other. Five months after conception the instan- 

 taneous percentage growth-rate is only 1-7 per day, while, during 

 the week preceding birth, the rat embryo grows at the rate of 53 per 



