EMBRYOLOGY OF CRYPTOBRANCHUS 87 



It thus appears that the paternal brooding instinct in both 

 species of Cryptobranchus is manifested from the moment the 

 eggs are fertihzed, though in C. allegheniensis at least it may be 

 temporarily inhibited or overcome by hunger or by the breeding 

 instinct. The brooding habit of Cryptobranchus is undoubtedly 

 very old, and we must look to other forms to find examples of it 

 in the incipient condition. 



According to Whitman ('98) there are three distinct elements in 

 brooding behavior: (a) the disposition to remain with or over 

 the eggs; (b) the disposition to resist and to drive away enemies ; 

 and (c) periodicity. The first of these elements has its origin in 

 the need for rest, protection to the offspring being at first inci- 

 dental. The second element, pugnacity, is periodical and a part 

 of the reproductive cycle. The third element, periodicity, is 

 apparently an attribute of the two other elements, based on physi- 

 ological conditions; its adaptiveness lies in correlating the other 

 two elements with the hatching period of the eggs. 



In Cryptobranchus, after spawning, the female is evidently 

 much the weaker of the two; as a matter of observed fact, she is 

 driven away by the stronger and more pugnacious male. It 

 can scarcely be the need for rest that keeps the male in the nest, 

 since he maintains exclusive possession at the cost of alert watch- 

 fulness and occasional combat. If the element of weakness were 

 the important factor in initiating the brooding habit, we should 

 expect the female rather than the male to remain in the nest. 

 Tt may be that primitively the brooding impulse is a phase of the 

 reproductive cycle that applies to both sexes, the female losing- 

 it on account of her hungry and exhausted condition due to the 

 accumulation of a large amount of yolk in the egg. Perhaps 

 (for this suggestion I am indebted to Professor S. J. Holmes) 

 on the part of the male there is involved a proprietary interest 

 in the nest, which he has chosen and in part excavated, and which 

 he occupies as an advantageous breeding place and as a more or 

 less permanent home. 



To obtain conclusive evidence regarding the origin of the brood- 

 ing habit one must study a series of closely related forms illus- 

 trating the habit in the making. In Cryptobranchus it appears 



