William J. Moenkhaiis 31 



times a clay and the eggs were allowed to develop as far as they would. 

 During the two-, four- and eight-cell stages the per cent of eggs impreg- 

 nated and the character of the impregnation was determined. The nor- 

 mally impregnated eggs were isolated and their further development 

 watched from time to time, and the desired stages preserved. 



The necessity of proper precautions is, of course, evident, to prevent 

 contamination by the introduction of other sperm than that desired. 

 These, in all the experiments, consisted in (1) carefully sterilizing all 

 the vessels and instruments that were used in the experiments; (2) 

 keeping the two sexes of the same species in separate aquaria; (3) care- 

 fulh^ washing the hands and the fish at the time of the experiment ; 

 and (4) carrying a control lot of eggs, taken from the same lot used 

 for the experiment, in a separate fingerbowl containing water from 

 the same source as that used on the eggs that were hybridized. I may 

 say here, that in none of the experiments did I find a single egg of 

 the control lots that showed any signs of development; so that there 

 is no doubt that in all of the experiments the possibility of an error 

 from contamination has been eliminated. 



Not all the lots of eggs that may be obtained during the spawning sea- 

 son of a species will show the same per cent of impregnation, even with 

 sperm taken from the same species. In order, therefore, to obtain a 

 more reliable estimate of the percentage of hybrid eggs impregnated, it 

 was essential in each experiment to fertilize a sufficient number of eggs 

 from the same lot to serve as an index of the normal condition. Fur- 

 thermore, it was most essential in determining whether the development 

 of the hybrids was going on under favorable conditions, to carry along 

 with them some normal eggs under the same conditions. 



The developing eggs were kept under close observation from the time 

 of impregnation to their death. Extensive notes were taken on the 

 living eggs and desired stages were preserved in a variety of killing 

 fluids — Perenyi's, Picro-acetic, Zenker's; and for surface study Child's 

 method, of first placing the eggs for about a minute in a corrosive-acetic 

 solution and then, after rinsing in water, in 10 per cent formalin, 

 was used. This latter method in both species of eggs leaves the egg 

 membrane and yolk beautifully clear while it turns the protoplasmic 

 portions white, thus making an ideal preparation for surface study. 



III. Nomenclature. 



For clearness and brevity's sake in the following discussion I have 

 found it desirable to adopt certain expressions which should here be 



