VARIATION IN ECHINOID PLUTEI 
A STUDY OF VARIATION UNDER LABORATORY CONDITIONS^ 
DAVID H. TENNENT 
Associate Professor of Biology, Bryn Mawr College 
TWENTY-ONE FIGURES 
No thoughtful naturalist will question the value of a knowledge 
of the normal development of a given form as a basis for experi- 
mental work. The necessity of a knowledge of the abnormal 
development is less generally understood. 
Our knowledge of the development of invertebrates is ol what 
we term the normal. By normal we mean that occurring under 
natural conditions and as a part of the course of events in the life 
history of any organism. Embryos which depart from this type 
are of little use to us as objects of study or as the basis of figures 
to be used in illustrating our monographs. 
In my observation the fate of an "abnormal" culture is imme- 
diate rejection. This treatment is of course what it should be 
if we desire only an acquaintance with the forms that show the 
least variation; with possibly the modal life history. 
In experimental embryology we need more than this. Our 
conditions of work are artificial. We must have an exact knowl- 
edge of development under these conditions. 
^ The observations upon which this paper is based were made at theUnited 
States Fisheries Laboratory at Beaufort, North Carolina, during the summer of 
1908. I am indebted to the Hon. George M. Bowers, Commissioner of Fisheries, 
for permission to work in the laboratory and to Mr. Henry D. AUer, Director of the 
laboratory, for the excellent facilities for work placed at my disposal. In the 
determination of the constants I have been assisted by Esther M. Tennent, who 
has made an independent calculation of all of the results obtained. 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 9, NO. 4. 
