1902.] NATUEAL SCIENCES OF PHILADELPHIA. 757 



It is evident from the above statements that Gonionema is exceed- 

 ingly sensitive to external conditions. Not all coelenterates are 

 affected in the same degree, and some are apparently not affected 

 at all by changes in illumination. Some medusse always lay their 

 eggs early in the morning, while others of nearly related genera 

 choose the evening or night. Experiments carried on by Wilson 

 and Donaldson under Prof. Brooks' direction, at Beaufort, N. C, 

 showed that in the case of Renilla and some sea anemones, at any 

 rate, changes in light and temperature did not affect the precision 

 with which the regular physiological processes took place. It is 

 well known that a great many marine animals show more or less 

 definiteuess in the habit of spawning. Metschnikoff gives a table® 

 showing the time of spawning of a large number of different genera 

 of jelly-fishes. In other groups the same tendency is manifest. 

 This phenomenon is probably the result of the working of natural 

 selection, the habit of laying the eggs at a certain definite time 

 having proved of value to the different species. The fact that in 

 some forms this precision of periodicity is not dependent upon 

 external influences, while in others there is manifest a marked 

 degree of sensitiveness to such stimula, seems to me to indicate 

 that the tendency has been arrived at by different processes, 

 and may be due to quite different requirements in the various 

 creatures. 



But to return to the dehiscence of Gonioyiema: not all the eggs, 

 by any means, which the ovaries contain are liberated at one time. 

 Medusse have been seen to deposit eggs every night for a week, and 

 while specimens kept in captivity are not very reliable in drawing 

 inferences as to natural processes, this period of sexual activity 

 would, it would seem, be more likely to be shortened than other- 

 wise by the unnatural conditions. After the first three or four 

 days on which spawning took place, a small number of ova were 

 left in the gonads, and on the three successive evenings these were 

 extruded a few at a time. Late in the summer the specimens 

 taken are usually devoid of sexual products, and the gonads small 

 and shriveled. 



C. Egg-Envelope. — In freshly laid eggs the polar bodies are 

 only rarely to be found. They are normally given off and lost in 

 the gonads previous to dehiscence. Before fertilization the eggs 

 ^Metschnikoff, 1886, Embryologische Studien. 



