ON EGGS OF MARINE ANIMALS 31 



around them. The practice of keeping the animals in a refriger- 

 ator can not be too severely condemned. The great excellence 

 of Nereis for experimental work is that every swarming individual 

 is always sexually mature, and contains no immature sexual 

 cells. 



Directions for obtaining eggs and sperm are simple. 



To obtain eggs, wash an isolated female by placing her in 

 clean sea-water, and snip her with sharp scissors. The eggs will 

 pour forth quickly. Remove the cut animal, wash the eggs by 

 pouring off the sea-water in which you had placed the worm and 

 add an equal volume of sea-water. I use 250 cc. of sea-water. 

 Now wash a male by placing him in 250 cc. of sea-water. Remove 

 and dry him lightly and quickly on soft filter paper. Place in a 

 clean dry Syracuse watch-glass and make a small cut about 

 half way between head and tail, along the lateral border to 

 avoid cutting the dorsal blood vessel. This gives you clean, 

 "dry," sperm-suspension, free from blood. For an insemination 

 add one drop of dry sperm to 10 cc. of sea-water. Of this 

 sperm-suspension, use two drops to the eggs of one female. 



Because of its almost clock-like precision of development, one 

 could scarcely wish for a finer object than the egg of Nereis. 

 If one does not get 100 per cent fertilization and almost perfectly 

 uniform rate of cleavage, one's technique is at fault. The 

 worker who believes in wide variability in the development 

 of eggs from one female should study the eggs of Nereis properly 

 collected and handled. Permit me to say that generalizations 

 on the wide variability among eggs from a female of a given 

 species are disproved if one uses animals in optimum condition 

 and handles properly their eggs and sperm. Nereis when 

 caught are always in optimum condition. The worker's results 

 therefore depend solely on the methods he employs after collect- 

 ing the worms. I make it a rule never to use Nereis that have 

 been in the laboratory more than sixteen hours. 



Platynereis diimerilii 



Platynereis dumerilii may be obtained in the heteronereid 

 phase at Naples throughout the year, as Ranzi has shown. 

 (Just, 1929, Ranzi, 193 i.) The methods for handling the eggs 

 and sperm are the same as those given for Nereis except that the 



