658 DAVID H. TENNENT 
In laboratory fertilizations the treatment of eggs and sperma- 
tozoa is from the first unnatural. The methods that we adopt are 
those that will enable us to imitate the natural sequence of events 
most closely. The successful culture is the one with the greatest 
number of embryos like those occurring in conditions of nature. 
All this is admirable, for its purpose. 
Suppose, however, that we wish to go a little further, that we 
desire to study the effect of a change in our artificial normal con- 
ditions. We then find ourselves puzzled in determining between 
the effect caused by the new element that we have introduced into 
the experiment and the appearance that may be the result of 
laboratory manipulation. 
In 1907 I began an investigation on Cross Fertilization in 
Echinoids. From my earliest experience as a student in a marine 
laboratory I had been familiar with the earlier development of 
most of the forms with w^hich I desired to work. 
The cross fertilizations were successfully made. I had no diffi- 
culty in seeing that a profound modification in structure had taken 
place, but from the first I found myself hampered by a lack of 
exact knowledge of the variation occurring in laboratory cultures. 
The observations described in this paper were the result of my 
attempt to supply myself with the needed information. This 
information gained at that time has since been of much use in a 
continuation of the study of cross fertilization. 
MATERIAL AND METHODS. 
The statistics given in this paper are based upon a study of the 
eggs and embryos of the sea-urchin, Toxopneustes variegatus. 
The breeding season of this echinoid extends throughout the 
summer, being at its height during June and July.^ 
2 During the three summers preceding 1908 and again in 1908 I noticed that the 
gonads of sea urchins taken after a night of full moonlight were empty, while those 
obtained a week later gave an abundance of eggs and spermatozoa. The first 
exception. to the rule was noticed in July, 1908, when after two nights of bright 
. moonlight the supply of Toxopneustes obtained was in exceedingly good condition. 
No observations as to the habits of the sea urchins could be made for the reason 
that they were obtained from water in which the bottom could not be seen. 
