Ill 



METHODS FOR HANDLING EGGS 

 AND SPERM IN THE LABORATORY 



Before I take up the discussion of individual species of eggs 

 and spermatozoa, I should like to speak briefly on some points 

 of general application. 



General Considerations 



A convenient classification of eggs is based on the stage of 

 maturation at which they are fertilizable. In this wise, all 

 animal eggs fall into four classes: those fertilizable before matura- 

 tion (in the stage of the intact germinal vesicle) ; those fertilizable 

 during first maturation; those fertilizable during second matura- 

 tion; and those fertilizable after complete maturation. Classes 

 I, 3 and 4 oiTer no difl^iculty for fertilization in the laboratory, 

 because the eggs are shed in the fertilizable stage. Class 2 

 comprises some species of eggs which are shed with the germinal 

 vesicle intact whose break-down is stimulated by the sea-water. 

 Here one must wait with fertilization until this break-down has 

 occurred. Some other eggs of this class are shed with the first 

 maturation spindle forming or formed; at this time they may at 

 once be fertilized. In handling eggs and sperm one must ascer- 

 tain to which of these classes the species of eggs employed 

 belongs. The addition of sperm to eggs not yet in their fertiliz- 

 able stage may often keep the eggs from attaining this stage. 

 (On this point see the discussion on the egg of Asterias below.) 

 In other cases, as for example in eggs of echinids, although the 

 spermatozoa induce changes in the eggs with intact germinal 

 vesicle or in stages of maturation, these eggs never develop. 



Another general point concerns the aging of eggs as they lie 

 in sea-water. This varies with the species. Some eggs, as 

 those of Platynereis megalops and of Jmphioxus, lose fertiliza- 

 tion-capacity very shortly after having come into sea-water. 



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