210 BULLETIN : MUSEUM OF COMPAllATIVE ZOOLOGY. 



The proportion of close fertilized eggs was greater in this experiment 

 than in the preceding. Many of the eggs so fertilized, however, never 

 developed beyond the 2- or 4-cell stage. The cross fertilized eggs all 

 developed normally and at the same rate. 



Combining the results of Tables III. and III. a, and those of IV. and 

 IV. a, we get an average of 7.7% of the eggs developing after close fer- 

 tilization, and 95 % developing after cross fertilization. These averages 

 agree fairly well with those obtained from Experiment 2, which were 

 4.8% and 94% respectively. 



Experiment 3 shows conclusively that, in the case of Ciona, eggs are 

 to a large extent incapable of fertilization by sperm from the same indi- 

 vidual as the eggs. Cross fertilization must, therefore, be the rule, and 

 close fertilization the exception under natural conditions. The rare 

 occurrence of close fertilization is probably due to a lack of mutual 

 attraction between eggs and sperm produced by the same individual, an 

 attraction invariably existing between the eggs of one individual and 

 the sperm of another, and probably chemical in its nature. This case 

 is paralleled in certain flowering plants, whose pollen will not germinate 

 when placed on the stigma of the flower from which it was taken, though 

 on the stigma of other flowers of the same species of plant it germinates 

 readily. 



There seems to be a particular time of day in the case of each species 

 of simple Ascidian for the discharge of the sexual products. Difterent 

 aquaria, in which are placed individuals of the same species, if they are 

 subjected to the same conditions of temperature, etc., invariably contain 

 eggs in exactly the same stage of development. This shows conclusively 

 that the time of egg-laying has been the same in the case of each 

 aquarium. For, on account of the rapidity of development, a slight 

 difference in the time of egg-laying would be readily detected by a dif- 

 ference in the stage of development exhibited by the eggs in different 

 aquaria. In the case of Ciona the sexual products are discharged about 

 an hour or an hour and a half before sunrise. The stimulus to their 

 discharge is probably the increasing light of daybreak. 



If at about the time mentioned one approaches the aquarium with a 

 lighted lamp, he will see the animals suddenly contract violently two or 

 three times in succession, then resume their accustomed tranquillity. A 

 careful examination will then reveal the eggs floating as little golden 

 specks in the thoroughly agitated water. Soon they begin to settle to 

 the bottom of the aquarium and can then be collected in convenient 

 quantities by means of a pipette. The violent expulsion of the contents 



