APPENDIX 



I 

 DIFFERENCES IN EGGS WORTHY OF NOTE 



For experimental work on animal eggs it is well to bear in 

 mind that the fertilization process varies depending upon the 

 stage of maturation in which fertilization normally occurs. 

 Eggs thus fall into four classes: 



Class I, fertilizable in the stage of the intact germinal 

 vesicle. Examples: Eggs of Nereis, Thalassema, Myzosio?na, 

 and Mactra. 



Class II, fertilizable in the stage of first maturation. 

 Examples: Eggs of Chaetopterus, Mytilus, Cumingia, Ciona and 

 Molgula. Eggs of the genus Asterias belong here also. 



Class III, fertilizable in the stage of second maturation. 

 Example: Egg of Amphioxus. 



Class IV, fertilizable after complete maturation. Examples: 

 Eggs of echinids. 



Too frequently experimentalists work on eggs of one fertili- 

 zation class and incorrectly draw conclusions concerning eggs of 

 other classes. Morphologically as well as physiologically, the 

 process of fertilization is different for each class. Animal eggs 

 even of the same fertilization class differ in other respects. 

 These differences also should be kept in mind. 



Thus eggs differ with respect to the presence or absence of 

 jelly, the distance at which the membranes separate after insemi- 

 nation, changes in shape of the egg after membrane-separation, 

 etc. Also, the egg may possess a micropyle through which the 

 sperm enters; lacking this, sperm entry may be at a fixed point 

 or not. A fertilization cone of larger or smaller size which per- 

 sists for a longer or shorter time, may or may not form. 



An egg at the moment of normal fertilization may or may 

 not possess an aster or asters in association with its nucleus in 



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