Prenatal Growth of the Human Body. 149 



in the head region, including also individually the brain, eyeballs, 

 and tongue. ^° 



Beyond this it is perhaps unsafe to generalize, on account of the 

 scanty data available ; but it seems likely that the viscera in general 

 (circulatory, r^iratory, alimentary and genito-urinary) are as a 

 rule relatively smaller in the adult than in the earlier stages of the 

 higher vertebrates. If this is true, it follows that the remainder of 

 the body (chiefly locomotor apparatus) must be relatively greater 

 in the adult, which appears to be true in the chick, dog and shrew, 

 as well as in the human species. 



Significance of the Growth Changes. 



It is the purpose of the present paper to present facts concerning 

 growth, rather than to speculate upon their significance.^^ It may 

 be worth while, however, to point out that growth can be considered 

 with reference to (1) its immediate causes, or (2) its physiological, 

 ontogenetic or phylogenetic significance. Concerning (2) we may 

 safely say that the size of an organ or part, like its position, form, 

 and structure, may depend u^pon, or be related to, the present, past 

 and future. The present refers to the physiological relations which 

 the part bears to the existing organism, an increase or decrease in 

 function being generally correlated with a corresponding increase 

 or decrease in size. The past refers to the conditions associated with 

 the ontogenetic^^ or phylogenetic history (of e. g.. Wolffian bodies) 

 of the individual. 



The future refers to changes in an organ which take place in antici- 

 pation of physiological needs which will arise at a later period in the 

 life cycle of the organism. Preyer's (39) statement that "Im em- 

 bryonalen Leben diejenigen Theile am schnellsten wachsen welche 



^"The prenatal tongue is relatively larger in the chick, dog and shrew (Welcker 

 and Brandt). 



"For a detailed consideration of the principles of embryonic growth, cf. His 

 (22). 



^^It is noted that, in general, organs which arise as infoldings (brain, spinal 

 cord) are relatively large at the beginning and decrease later, while the con- 

 verse is true of organs which arise as outgrowths (glands, lungs, etc.). 



