EGG-VOLUME AND FERTILIZATION MEMBRANE. 57 



induce parthenogenesis) the data still furnish one or two points of 

 further interest. 



The shrinking of the eggs of Echinarachnius eccentrica when 

 subjected to optimum hypertonic solution for parthenogenetic 

 development was from the mean diameter 13.27 before, to one of 

 1 1.6 mm. at the end of the treatment, or a reduction of 12.6 per 

 cent. After return to normal sea-water, and by the time cleavage 

 completed the 2-cell stage, the mean diameter was 15 mm. an 

 increase of 13 per cent.; by the time the eggs were in stages 

 ranging from 4-cell to morulae the mean diameter was 16.2 mm. 

 showing a total increase of 22 per cent. 



In the case of the inseminated eggs of this species the fertili- 

 zation membrane (assuming it to be present on the unfertilized 

 egg) showed an increase, while developing to the 4-cell stage, from 

 a mean of 13.27 to one of 16.3 mm. an increase of 22.8 per cent. ; 

 eggs in stages of 8-cells to blastulae showed an increase to a mean 

 diameter of 16.75 mm. or a mean total increase of 26.2 per cent. 



In the case of Asterina miniata the increase in the average total 

 diameter of fertilization membrane of inseminated eggs is from 

 19.21 to 21.0 scale divisions or one of 9.3 per cent, comparing 

 averages the increase is 16.8 per cent. ; of the acid treated parthe- 

 nogenetic eggs the increase of both egg-cytoplasm and fertilization 

 membrane (as in the case of hypertonic parthenogenetic eggs of 

 the sand-dollar) is equal, and is the difference between 19.25 and 

 20. o mm., or an increase of 3.9 per cent. 



5. It may be of further interest to calculate the approximate 

 mean actual diameters and actual volumes 6 of these eggs. If we 

 assume them to have been spheres in all cases we have the 

 following: The mature unfertilized egg of Echinarachnius 

 eccentrica has a mean diameter of 120 ji, from which its volume 

 must be about .0009 mm. 3 ; the optimum hypertonic shrinking 

 gave a diameter of 105 /*, or a volume of .00061 mm. 3 , representing 

 a volume reduction of 33 per cent. 



The mean actual diameter of mature Asterina miniata eggs is 

 174.6/1, representing a volume of .00278 mm. 3 ; the mean di- 

 ameter of egg-cytoplasm just after fertilization is about 161 M, 



6 These volumes are calculated by multiplying the cube of the radius by 4.18 in 

 each case. The radius is found by dividing the mean projected greatest diameter 

 of the egg (that given in the text) by 2 x no, no being the magnification. 



