Variability of Eggs and Sperm of Sea-Urchins. 77 



cent of the eggs, 2 drops only 14 per cent, 8 drops 20 per cent, and 16 

 drops 46 per cent. The increase was roughly proportional to the 

 increase in sperm. In 6/1 7s 1 drop of standard sperm suspension 

 gave only 1 per cent cleavage, 16 drops caused 75 per cent to cleave. 

 In 6/1 7 A maximum cleavage occurred with 8 drops of sperm suspen- 

 sion; 16 drops of the same suspension made the solution milky. The 

 matter was tested in other ways. All observations pointed to the 

 conclusion that 15 to 80 times as much sperm was required to produce 

 optimum results in Hipponoe as hi Toxopneustes. 



ARBACIA. 



F. R. Lillie's careful researches with Arbacia have made known the 

 optimum concentrations for this species. My observations corroborate 

 his in every detail. Having ascertained the optimum conditions of 

 concentration for a given surface and volume of sea-water, for each 

 species of egg, I then undertook the study of their normal variability, 

 which will be considered under the following headings: size and shape; 

 jelly layer; membrane formation; cleavage. 



VARIATION IN SIZE AND SHAPE OF EGGS. 



In a sample lot, eggs were measured consecutively with an eye-piece 

 micrometer (1/3 eye-piece, 1/6 objective); much care was exercised to 

 avoid changes in size from pressure of cover-glass and to avoid errors 

 due to poor focusing. The readings give relative values only. Re- 

 peated measurements gave so nearly the same readings that the small 

 number of eggs cited in the table may be taken as a very close approxi- 

 mation to the actual situation. 



Table 3 shows the extent of variation in size in the eggs of Hipponoe. 

 It shows the considerable variation in the maximum, minimum, and 

 modal size of batches of eggs from different females collected at the 

 same tune. The variation, often as great as in females collected at 

 different times, is a result either of a general enlargement or less 

 frequently of a shrinkage from the norm of the species. Such change 

 in size indicates so-called "poor" eggs and is correlated with other 

 evidences of poor physiological condition. 



Similar results were obtained with Arbacia eggs, though the maxi- 

 mum mode and minimum are different. 



SHAPE OF EGGS. 



I have no actual counts of the variation in the shape of the eggs. 

 Most fresh eggs are globular, though not infrequently a considerable 

 number of the eggs are distinctly elliptical, in rare instances angular. 

 While I have no records of the numerical relation of these three types 

 of eggs, which intergrade, yet the presence of these three groups and 

 the considerable variation in different females is undoubted. 



