512 



H. H. Newman 



3 The Importance of Equalizing the External Conditions of the 

 Developing Embryos 



After some experience it was found necessary to limit the num- 

 ber of eggs in one vessel and to keep these well separated. The 

 tendency to clump up, exhibited by the eggs of F. heteroclitus, 

 causes marked variations in time rate of development and ulti- 

 mate success in hatching. If eggs are too numerous or too crowded 

 in one vessel only a few succeed in hatching and many abnormal 

 conditions are manifested. F. majalis embryos do not endure 

 crowding so well as do those of F. heteroclitus, so, as a rule, only 

 about half as many of the eggs of the former species are used in 

 an experiment as those of the latter species. 



Several methods were used for keeping the vessels well aerated. 

 Running sea-water was used, but it was found that a rusty deposit 

 collected on the surface of the eggs. This deposit seemed to 

 interfere with development; probably interrupted the gaseous 

 respiratory exchange. Aquaria containing Ulva were used in 

 some of the experiments, but these did not prove very satisfactory 

 on account of the fact that the eggs lay still too long and frequently 

 became covered with Saprolegnia and other molds. The method 

 that gave the best results was simple. Large, flat, covered bac- 

 teria dishes were used and the water was changed daily by 

 pouring off the surface and filling up with fresh sea-water. In 

 this way disease was kept at a minimum and there was always an 

 abundance of well aerated water. 



It is absolutely necessary to pick out all eggs that have not been 

 fertilized as well as embryos that die from time to time, as these, 

 if left in the vessels, will infect the healthy individuals. 



4 The Attitude that Must be Taken Toward Variability 



Another lesson taught by experience in dealing with large 

 numbers of eggs and embryos is, that the high degree of variability 

 in different strains and in different individuals of the same strain, 

 must not be regarded as an insurmoutable difficulty in an endea- 

 vor to arrive at definite results. Variability, here as elsewhere, 

 is to be expected, and one must accept it as he finds it and must 



