1S4 University of California Publications. [zoology. 



The interest in the question of the changes that take place in 

 the ectoderm during larval life lies in the relation of this question 

 to the large problem of growth and development of the larva. 

 It appears that the difference in size and form assumed by the 

 larva at different times in its career is more a question of the 

 distribution of a nearly constant quantity of body substance than 

 of the addition and distribution of m :w substance. Morgan, '!)4. 

 was the first to call attention to this matter. He did not, how- 

 ever, any more than we have, bestow upon it the searching study 

 it certainly deserves. We have not sufficient data, particularly 

 as to the morphology of the first stages of development, and as 

 to the food taken during larval life, to make a discussion of the 

 fundamental problems involved profitable at this writing. We 

 consequently are obliged to leave the subject for the present and 

 rest content with having pointed out that the redistribution of 

 substance in the ectoderm that takes place during metamorphosis 

 is not a process, primarily at least, of the degeneration and 

 resorption of the thinnest parts of the ectoderm, but rather of 

 growth in the thin parts even more active than in the thicker 

 parts. Morgan, '94, p. 61 has recognized stages, or periods, in 

 the life of the tonaria, though he does not mark them off into the 

 three here indicated, and in one respect our observations are at 

 variance with his conclusions in a rather important point of fact. 

 During the second phase of larval growth, he says, a continuous 

 decrease in size takes place, "but," he continues, "it is 'interest- 

 ing fa note that no new organs are formed while the larva is 

 decreasing in site." This last statement does not hold for the 

 present species, for, as we have pointed out, at least one 

 important set of new organs, namely, those of respiration, are 

 initiated during this period. The first pair of gill pockets appear 

 almost simultaneously with, or at least but slightly after, the 

 beginning of diminution in size of the larva; or, iu other words, 

 almost coincidently with the beginning of resolution of the 

 tentacles. We must recognize, too, the differentiation of the 

 proboscis as being begun in this period. The very initial 

 change toward the formation of this region of the body is seen 

 in the beginning of the longitudinal musculature, of the pro- 

 boscis. It has been abundantly shown by other writers that 



