LARVAE OF ARBACIA PUNCTULATA 



333 



had elongated to 58.25, an increase of 42.94. This represents 

 the period of most rapid growth. 



During the next 24 hours a retarding in rate of skeletal de- 

 velopment took place, and at the end of 72 hours the embryo 

 measured 73.62, an increase of 15.37 or about one-third of 

 that obtained the previous day. During the next 24 hours rate 

 of growth diminished still more, and, at the close of this period, 

 the larva had reached 78.68, an increase of only 5.06, or 6.8 

 per cent. 



At the age of 120 hours the larva had attained its maximum 

 average length, 80.34, which is only 2.1 per cent in excess of 



Plot 1 



that 24 hours previous. At this time, however, a wide varia- 

 tion prevailed among the larvae of different cultures. Some 

 were just attaining their greatest length while others had already 

 passed this point and had begun to undergo reduction. As 

 resorption of the skeleton took place very rapidly, individuals 

 of this age were found with anal arms almost completely lacking 

 and oral arms scarcely more prominent (fig. 24). 



It was evident, not only from observation of the activity of 

 the sperms and of the percentage of mature females, but also 

 from a comparison of the growth-curves of these periods, that 

 the individuals raised from eggs of the early period were much 

 more vigorous. Their development during the first 24 hours 

 was more rapid, and they attained a much greater size. Those 

 obtained during the second period of 1914 were much less 



