254 Experimental Zoology 



way in Minot's experiments with pregnant guinea pigs not fully 

 grown. Not only did the mother grow at the normal rate, but 

 she supplied the embryo also with a large amount of nourish- 

 ment. 



We can account for the facts on the assumption that when less 

 than the optimum is given more substance is digested from the 

 food, and when more than the optimum is taken proportionately 

 less is absorbed or assimilated. The appetite of the animal fur- 

 nishes in a measure an index of the amount needed for growth 

 and repair, but one that cannot be entirely relied upon. 



The size of the adult stage of some of the lower animals is 

 much affected by the amount of food that can be obtained. 

 There is, however, an upper limit that is quickly reached when 

 food is abundant that cannot be surpassed. Most of the varia- 

 tion in size lies below this condition. For example, fresh-water 

 planarians if kept without food for several months decrease in 

 size, and finally may be not more than about 1/15 of the original 

 volume. If such a starving worm is cut in two, each piece still 

 has the power to regenerate the missing part, drawing on or 

 making use of the starving tissues in order to make the new 

 growth. Thus, although the worm is starving it will grow rela- 

 tively rapidly at the cut surface and produce a new part. If 

 a starving earthworm is cut in two in the middle, the anterior 

 piece makes a new tail, and the posterior piece also regenerates 

 a tail (reversed in direction) at the anterior end of the piece. 

 In the latter case the old part continues to waste away, while 

 the new reversed tail continues to add new segments to its grow- 

 ing end. Thus while starvation is taking place in the old piece, 

 growth goes on in the new, and the latter must derive all of its 

 material from the starving portion. In the "winter" salmon of 

 the Rhine the tissues of the body are used as food after the fish 

 have entered fresh water, when they cease to feed. These sal- 

 mon may live for months (from 8 to 15, according to Meischer) 

 without food. At the end of this time the fish are much emaci- 

 ated. During the period of starvation the eggs develop. There 

 can be little doubt that a starving mammal, if pregnant, would 



