22 BASIC METHODS FOR EXPERIMENTS 



ments eggs and sperm that are abundant and easily available. 

 If in addition such eggs are universally popular among experi- 

 mental embryologists, we ought to have some simple directions 

 for handling them. 



Arbacia 



The worker may obtain eggs and sperm of Arbacia in opti- 

 mum fertilizable condition by one of several methods: (i) allow- 

 ing the animals to shed; (2) cutting carefully around the 

 peristome, without injury to the gonads, or removing the spines, 

 either of which stimulates shedding; and (3) cutting around the 

 equator of the test and removing the ovaries to 250 cc. of sea- 

 water, the testes to a dry watch glass. 



(i) Freshly collected ripe Arbacia readily shed their sexual 

 products. Indeed, this may be a nuisance if the worker has 

 among several such animals in a tank one that starts shedding. 

 I have frequently observed an animal on the side of a tank 

 begin shedding; next, the adjacent one sheds and so on around 

 until soon from every ripe animal eggs or sperm stream forth. 

 Similarly, I have found thick mats of eggs lodged among the 

 spines of females on the bottom of the tank. Elsewhere (Just, 

 1922) I have commented on the fact that I have taken perfectly 

 normal eggs in various stages of development from among the 

 spines of living females. I have also collected, from the piling 

 of the wharf opposite the Crane building at Woods Hole, Arbacia 

 that shed before I could get them to the laboratory, a distance 

 of about one hundred yards. 



These shed eggs are of optimum fertilizability except toward 

 the end of the breeding seasons (Just, 1922). Since one could 

 scarcely depend on shed eggs for one's experiments, one must 

 use other methods for obtaining eggs in optimum condition. 

 I therefore suggest the following methods. 



(2) Eggs shed as the result of stimulation through injury to 

 the animal, e.g., by cutting carefully around the peristome — ■ 

 without puncturing the ovaries — or by removing their spines, 

 are by no means inferior to eggs normally shed, as their per- 

 centage and normality of development reveal. The animals 

 thus stimulated should be placed aboral surface down in clean 

 dry Syracuse watch-glasses, and the eggs so obtained ("dry 



