LARVAE OF ARBACIA PUNCTULATA 319 



of experimentation, it was hoped to obtain some data on the spe- 

 cific effects of the various factors in the environment and on the 

 laws governing the reactions of the embryos to these fluctuating 

 conditions. And finally, by determining the limits to which 

 variations could be pushed in the developing embryos, it was 

 hoped to ascertain the probable extent of variations in the 

 larvae of Arbacia punctulata in their natural environment. 



LITERATURE 



Studies of limited phases of this problem have been made by 

 previous investigators on Arbacia and on other American and 

 European species, mainly Toxopneustes, Strongylocentrotus, 

 Echinus and Spaerechinus. 



Tennent ('10) made a study of the eggs and embryos of Tox- 

 opneustes variegatus with a view to obtaining more detailed 

 knowledge than had ,been previously recorded, of the develop- 

 ment under laboratory conditions. The variations observed by 

 him "fall into three classes, — fluctuating variations, defects, 

 multiplicities. The fluctuating variations were those of length 

 and correlation," and were determined by the statistical method. 

 The defects consisted in the absence of a skeletal rod or of an 

 arm, or of a malformation of a part of the body; the multiplici- 

 ties in the presence of more than the normal number of skeletal 

 rods or more than the usual number of arms. The frequent 

 appearance of some characteristic defect or multiplicity among 

 the embryos of a series of cultures in which the eggs or the sperm 

 were all derived from a single individual, offered an opportunity 

 for the study of inheritable variations. 



Vernon ('95 and '00) also applied statistical methods as being 

 preeminently suited to a study of variations. In these two 

 papers he attempted to determine not only the range of varia- 

 tions but also their causes: the effect, if any, that changes in 

 environment may produce, and the period in the life of the 

 embryo at which it is most susceptible to modifications in 

 environmental conditions. He established "that Echinoderm 

 larvae vary from the average in respect of size of their various 



