14 BASIC METHODS FOR EXPERIMENTS 



mature worms with fully ripe sexual products swim. Thus one 

 may be sure of the perfect condition of the gametes. This is the 

 great advantage of working with fhe egg of these and similar 

 forms. 



If one desires to carry on experiments that demand normal 

 larvae, the distribution of the oil drops furnishes a criterion of 

 value for three species of Nereis: I found that the normal trocho- 

 phore shows one and only one oil drop in each of the four cells 

 of the gut. (Just, 1922^.) The egg of Perinereis at Roscoif, 

 more hardy than that of any other nereid egg known to me, is in 

 this respect similar to Nereis eggs: its larvae show, if they are 

 normal, the same distribution of the oil drops. 



In Asterias eggs 



The eggs of Asterias are normal only if after having come into 

 sea-water their germinal vesicles fade at the same rate. This 

 rate depends upon temperature, provided the eggs are suspended 

 in sufficient volume of sea-water. Contrary to the belief of 

 many workers, this egg is in the optimum condition for fertiliza- 

 tion after the break-down of the germinal vesicle and before the 

 extrusion of the first polar body. Work done on the egg of 

 Asterias after complete maturation is work done on an egg no 

 longer in optimum condition. 



In other eggs 



Echinid eggs represent one, and nereid eggs represent the 

 other of the extremes with respect to the time when in their 

 development eggs are fertilizable. That is, echinid eggs are 

 fertilizable only after complete maturation and nereid eggs are 

 fertilizable only in the germinal vesicle stage. For eggs like 

 those of Chaetopterus, whose germinal vesicles break down after 

 the eggs come into sea-water, the criterion for normality obvi- 

 ously is the appearance of the first maturation spindle. 

 Undoubtedly, one could ascertain experimental tests compa- 

 rable to those detailed above which would serve as valuable 

 indices for normality. These have yet to be devised. Eggs 

 fertilizable in the stage of second maturation, as for example 

 those of Amphioxus, also would reveal criteria by which one 

 could establish prognostics of subsequent normal development. 



