410 GRACE MEDES 



two centers, which have grown very S3minietrically as regards 

 each other. 



Figure 141 represents a specimen from culture / at an age 

 of 96 hours. Here two secondary skeletons are present, that in 

 the right having developed most of the characteristic parts 

 while that in the left side has not grown beyond the tri-radiate 

 stage. 



This production of radial symmetry is the most characteristic 

 change caused by the employment of carbon dioxide. Herbst 

 obtained it in Spaerechinus and Echinus b^ reducing the amount 

 of potassiimi or sulphate in the medium. In the latter case, the 

 number of skeletal centers sometimes reached seven. With 

 the addition of lithium halides he obtained a similar effect, 

 which resulted in the appearance of three, four or five centers. 



Fischel obtained radial symmetry by the employment of 

 hypertonic KCl solutions, by which also the number might be 

 increased to five. It seems difficult to reconcile the fact that 

 absence of potassium and hypertonic solutions with an excess of 

 potassium could produce the same result. Herbst suggests that 

 the reduced larval size in K-free sea-water may be due to a 

 failure on the part of the tissues to imbibe water in sufficient 

 quantities. If this in any way affects the symmetry, we should 

 expect the same results to be obtained with hypertonic solu- 

 tions. This, however, does not occm- in Fischel' s other experi- 

 ments nor is any evidence of it to be observed in the experiments 

 with hypertonic solutions described above. 



The appearance of an uneven number of skeletal centers can- 

 not be due to mere doubfing of that of one side. Herbst says, 

 "1st nur ein Dreistrahler vorhanden, so kann man wegen der 

 unregelmasigen Lage nicht sagen, ob dies der rechte oder linke 

 ist, und ebensowenig kann man bei Vorhandensein von dreien 

 behaupten, dass der linke oder der rechte verdoppelt sei." He 

 describes the gradual appearance of bilateral symmetry thus; 

 "Man kann aber trotzdem als Regeln bezeichnen, dass zwei 

 von den Dreistrahlern, deren Lage mehr an die Norm erinnert, 

 die anderen in der Entwickelung tiberholen so dass schliessUch 



