ON EGGS OF MARINE ANIMALS 21 



Others, as those of Nereis limbata and of Arhacia, remain fer- 

 tilizable for a longer period. Eggs which withstand residence 

 in sea-water, lose this resistance at higher temperatures and after 

 repeated washing with sea-water. One must, therefore, know 

 the capacity of the egg which one uses to resist the action of 

 sea-water. It is best to use eggs freshly shed or removed from the 

 animals and to avoid the use of stale eggs. Staling of eggs is 

 itself an experimental procedure used both in straight and 

 especially in cross-fertilization. 



In general, sperm-suspensions should be freshly prepared 

 because spermatozoa, especially in attenuated suspension, lose 

 viability in sea-water. Even actively motile sperm may be of 

 low fertilizing power. Where sperm are inactive, one must 

 determine the cause of this inactivity. If it is due to imma- 

 turity, the sperm-cells will not fertilize eggs. Only mature 

 spermatozoa are capable of fertilization. Where sperm are 

 removed from the males, one must establish whether or not they 

 are ripe. Unripe sperm often appear in bundles. Spermatozoa 

 of squid are in spermatophores; these should not be removed from 

 the animal until the moment when fertilization is to be made. 

 At this time the spermatophores are brought into sea-water 

 which induces the discharge of the spermatozoa. Decapod 

 (Crustacea) spermatozoa explode singly in sea-water, as Kolzoff 

 has shown. 



Notes on Special Forms 



There follow now some notes on particular forms. This 

 emphasis on the eggs of these species does not of course imply 

 that all experimental work should be done on them. On the 

 contrary, some problems in experimental embryology can be 

 best attacked by study of eggs not detailed beyond. I do not 

 mention eggs of sponges and coelenterates; nevertheless, they 

 are admirable for certain researches and ought be studied more. 

 In certain flat-worms one can follow under the microscope the 

 process of egg-laying, the escape of the tg^ from the ovary, 

 the spinning of yolk around it, and the secretion of the shell 

 that encloses it — a fascinating observation. These eggs also 

 have been too greatly neglected by experimental embryologists. 

 Nevertheless, a certain advantage adheres in using for experi- 



