336 William E. Kellicott 



quite largely upon the constantly varying nutritional conditions of the 

 fish. 



The somewhat similar form of the growth curves of the gonad weights 

 must of course have a totally different significance. The gonads (Plate 

 7), while at first like the other viscera, are unique in showing later both 

 an absolute and a relative increase in weight throughout life ; this occurs 

 earlier and is more pronounced in the testes. The gonads are the only 

 organs whose curves of growth give evidence of being composed of two 

 overlapping cycles : in the males a second growth C3'Cle commences when 

 a total weight of 800 to 1000 grams has been reached, and in the females 

 when the total weight reaches 2000 to 3000 grams. The periods at which 

 this second increase in size of the gonads occur, namely, of sexual maturity 

 in each case, give a clue to its cause and real nature. 



The first cycle which is in progress at birth, is that during which these 

 organs reach their primary maximum soon after birth ; this probably 

 is equivalent to the only cycle present in the growth of the other organs 

 and occurs at about the same time. During this cycle the gonads are 

 not reproductive in function; they are growing as more or less undiffer- 

 entiated mesodermal organs, not wholly undifferentiated, for they must 

 be producing internal secretions which affect the rate of growth of 

 certain other parts in such a way as to form at any rate the secondary 

 sexual characters. As this phase of growth commences to wane just 

 as it does in the other viscera, the gonads become truly reproductive 

 in structure and in function and as such organs they enter upon a second 

 cycle of growth which carries the size of these parts to a much higher 

 point, and which is never completed in the sense that it reaches a maxi- 

 mum and then declines during life. This phase of growth is more pro- 

 nounced in the testes than in the ovaries because the latter are not so 

 largely composed of strictly reproductive tissue, but contain relatively a 

 much larger amount of connective tissue. 



We have already mentioned the apparently composite character of the 

 latter part of the growth curve of the ovary. These weights were being 

 determined during the season of ovulation and in Plate 7 the curves A 

 and B show respectively the average weights of the ovaries before and 

 after ovulation. There must be consequently a regular rhythm in the 

 weight of the ovary which extends over a period of one or two years — 

 it is not certain whether this dogfish can produce young annually or only 

 biennially, although probably the latter is the case. The curve of ovaries 

 which have recently discharged their ova drops quite to the horizontal. 



