LARVAE OF ARBACIA PUNCTULATA 



397 



although NaCl is present in highest quantities in sea-water, the 

 effects of concentration by evaporation are not merely specific 

 for NaCl but are complicated by the presence of the other con- 

 stituents, which are increased in the same proportion. 



Figure 105 is a drawing of a specimen from the control at 

 48 hours and figure 106, of a specimen from solution h. In the 

 latter there occurs a fusion of the posterior body-rods with 

 developing processes, in contrast to the simple unfused condi- 

 tion of the corresponding elements in a. The arm-rods, on the 

 contrary, are still separate, and fusion is only slightly more 

 advanced in 108, which represents a specimen from the same 

 culture 72 hours old. In figure 109, c '^2 hours, the char- 

 acteristics depicted are the same but are more pronounced. 

 Figures 111 and 112 are taken from h and c at the age of 96 

 hours. In the latter, normal fusion of the arm-rods has taken 

 place but the long spine-like processes are conspicuous. Fig- 

 ures 114 and 115 represent the same cultures at the age of 120 

 hours and in most respects are characteristic. In solution d 

 (fig. 116), inhibition has been so great that a true pluteus form 

 is not attained by the embryo and only the rudiments of arm- 

 processes are acquired. This is in marked contrast to the re- 

 sults of experiments in which isotonic NaCl solutions are sub- 

 stituted for portions of the sea-water, as in the latter case, body^ 

 size is greatly decreased in proportion to arm-length. 



The hydroxyl ion, the carbonates and the sodium, are all 

 studied in detail by Herbst ('03), together with the other con- 

 stituents of sea-water, in an effort to determine the exact role 

 of each in development. In investigating this problem, he em- 

 ployed artificial sea-water from which the element under con- 



