206 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



male and female sexual elements at different periods ; although in some 

 cases, where the sexual products mature simultaneously, self-fertilization 

 is not excluded. Neither of these statements holds good for Ciona. 

 Although in the adult period it produces both sexual elements through- 

 out the spawning season, and discharges them simultaneously, self-fer- 

 tilization rarely occurs, — a conclusion to which I have been led by 

 repeated experiments. The most complete series of these experiments 

 will be briefly described. 



The observation had been made that an individual accidentally left 

 overnight in an aquarium by itself laid eggs which failed to develop ; 

 whereas, when two or more individuals were placed together in an aqua- 

 rium, all other conditions being the same, the eggs laid developed almost 

 without exception. 



First Experiment. 



Acting on the suggestion thus offered, I placed together in an aquarium 

 two or three small, clear individuals (evidently young) ; in another aqua- 

 rium was placed an equal number of very large-sized (old) individuals. 

 A greater number of eggs was laid by the large individuals, as one would 

 naturally expect, but the eggs in both aquaria were perfectly fertile. 

 This experiment showed that ripe eggs and sperm are produced both 

 by young and by old individuals. 



Second Experiment. 



Twenty rather large-sized individuals were selected for experimentation 

 and divided into two lots, A and B, of ten individuals each. The animals 

 of each lot were carefully washed and placed in clean glass dishes filled 

 with fresh sea-water. The individuals of lot A (Table I.) were placed 

 each in a separate dish, those of lot B (Table II.) were placed two in a 

 dish. The next morning a careful examination of each aquarium was 

 made to determine what proportion of the eggs laid had been fertilized. 

 The experiment was repeated on five successive days ; on the sixth day, 

 as a control experiment, the lots were interchanged, the animals of lut 

 A (Table II.) being paired, and those of lot B (Table I.) isolated. The 

 results for the six days are embodied in Tables I. and II. 



Taking an average of the fifty-eight cases in which eggs were laid by 

 isolated individuals (Table I.), we find that 4.8% of the eggs were fertilized. 

 The occurrence on a single day in two cases of fertilization of 90% of the 

 eggs laid makes me suspect that the dishes were not properly cleaned 

 on that day, and that live spermatozoa ma}' have remained clinging to 

 the sides of the dish after the previous day's experiment. If so, and if 



