Growth of Brain and \'iseL'ra in Dogfish 333 



during a certain early period, the exact time and extent of wliicli may 

 vary, the hrain, heart, rectal gland, pancreas and spleen grow at a faster 

 rate than do the other parts of the body: and that after this period 

 these organs grow at a slower rate than the other parts. As other parts 

 we must include chiefly the muscular and skeletal elements, gonads and 

 probably fat. The conditions of the excretory system and of the periphe- 

 ral parts of the circulatory and nervous systems are unknown in this form, 

 but these constitute relatively small parts of the total weight with which 

 these other organs are being compared, and in forms where something 

 is known about them they too grow more slowly. The alimentary canal 

 seems to share this relation and in the dogfish its length increases at a 

 constantly diminishing rate. 



It appears therefore that while the various tissues or organs grow 

 at different rates, those forming the greater part of the bulk of the body, 

 namely, the locomotor and supporting tissues, continue throughout life 

 to increase in mass at a more rapid rate than do the brain, heart and 

 chief viscera. Ultimately, of course, were this relation continued, a 

 condition would be reached where these parts mentioned would become 

 physiologically incompetent to carry on the work of the constantly 

 increasing mass of the body. That is, the muscles and connective tissues 

 actually outgrow their controlling and metabolizing sources, the phys- 

 iological balance of the organism becomes unstable and death may result 

 naturally or the animal ma}^ become an easy victim to adverse circum- 

 stances. 



Many features of the curves are partly explainable by assuming that 

 the mass of an organ is related in a general, though not a precise, way 

 to the extent of its functional activity. Thus at the time of birth the 

 relative size of the heart and digestive glands is increasing rapidly and 

 this increase continues for a time after birth, while the young fish are 

 undergoing the most critical period of their existence and are making 

 the most numerous and rapid adjustments of their lives. The young 

 contained in the oviducts of the mother are almost inactive and food is 

 supplied abundantly and in a condition which permits of its easy 

 assimilation. Immediately after birth the small fish must capture and 

 prepare for assimilation their own food and the activity of the neuro- 

 muscular and digestive tissues is enormously increased. Probably the 

 relatively immense size of the brain at birth is a provision for this 

 period which comes on so abruptly, and the initial phase of rapid increase 

 in relative size of this organ which we have noted is absent from our 



