246 CHARLES R. STOCKARD 



ity during the postnatal period as are found to apply in the 

 embryo. The important question at once arises as to whether 

 there are periods during which starvation might produce no sub- 

 sequent ill effects, alternating with more or less critical periods 

 during which a similar treatment might be followed by consider- 

 ably more serious results. For example, if a given treatment be 

 administered to one group of animals from the second to the fifth 

 week after birth, and to a similar group from the fourth to the 

 seventh week, the results might be the same or they might be 

 very different. The results would depend very largely upon 

 whether significant tissue changes susceptible to variations in 

 developmental rates had occurred during the time intervening 

 between the two experunents. 



The interaction or competition among the growing and devel- 

 oping organs found in the embryo certainly continues during 

 postnatal development. The suppression of development in 

 certain organs and tissues by the activity of another organ is 

 splendidly illustrated by the glands of internal secretion. The 

 further development of certain secondary sexual characters, such 

 as hair and plumage, after subdued activity of the gonads is a 

 case in point. 



The difference in importance between two developmental 

 moments in the postnatal individual is, however, of far less sig- 

 nificance than in the early embryo, just as arrests during early 

 cleavage stages are of more far-reaching consequence than similar 

 arrests after gastrulation has occurred . Differences in developmen- 

 tal rate during postnatal periods incline to affect the finer features 

 or the type of the individual rather than cause actual malformation 

 or pathological deficiencies in the tissues. Such effects are read- 

 ily observed in many of the arrested, status, and infantile human 

 types. A fuller conception of the significance of developmental 

 rate and rhythm in the determination of human types and ap- 

 pearances will be given in a separate communication. 



