34 BASIC METHODS FOR EXPERIMENTS 



during July. Animals kept one to a dish shed rapidly. It is 

 more convenient to use the -smaller specimens placing them in 

 finger bowls; larger animals usually shed more eggs but need 

 larger dishes. The eggs are useless if taken from the ovaries. 

 The animals are best kept in wet sand after collecting and 

 should be protected from rise in temperature. 



Mytilus 



The eggs of this form can be obtained in very large numbers. 

 They too resemble the eggs of Cumingia. For the worker who 

 desires more eggs from one female than he can procure from 

 Cumingia or Ensis for experiments similar to those for which 

 he used Cumingia eggs, here is the animal to use. There is the 

 drawback, however, that the animals must shed the eggs; eggs 

 taken from the ovaries are impaired. The abundance of Mytilus 

 makes it easy to get eggs throughout the breeding season. This 

 egg was studied by Hertwig and more recently by Meves (191 5). 

 The reader should consult Field's monograph, "Biology and 

 Economic Value of the Sea-Mussel, Mytilus edulis.''^ (1923). 



It is not at all difficult to carry this egg through meta- 

 morphosis. Indeed, Mytilus used to cause a great deal of 

 trouble in the old sea-water tank at the Marine Biological Labo- 

 ratory, Woods Hole. Larvae developed in such numbers that 

 the mussels Interfered with the water supply. I have had young 

 clams in diatom cultures a year old; these had developed from 

 veligers. 



Mya 



The eggs of Mya are obtained by allowing the animals to 

 shed. At Woods Hole, eggs are plentiful during the summer. 



Mact 



ra 



Unlike Cumingia^ the eggs of Mactra are fertilizable in the 

 germinal vesicle stage, thus resembling the eggs of Nereis and 

 Ascaris. This makes it an Interesting form for work on fertili- 

 zation and experimental parthenogenesis. For the latter point 

 especially see Kostaneckl's papers (1904 and 191 1). 



Eggs of Mactra differ from those of Cumingia and of the 

 other clams mentioned in still another way — they fertilize 



