FERTILIZATION IN PLATYNEREIS MEGALOPS. 95 



I have not been able so far to determine any structural differ- 

 ences in the ovocytes with and without sperm attached. In the 

 case of the eggs that maturate with the cortex wholly or partially 

 intact, the spindle may be abnormal. In most cases if it reach 

 the periphery of the egg it does so at a point practically devoid 

 of cortical cytoplasm. Or again, it may lie parallel to a tangent 

 of the egg membrane. 



Those sections which reveal the sperm within the egg are in 

 the minority. It appears from experiments several times 

 repeated during the four seasons of study that the penetration of 

 the sperm depends upon the amount of sea-water used. If the 

 eggs be inseminated in a large quantity of sea-water or washed 

 (by changing the water several times) very few eggs form jelly. 

 With less water more form jelly. Eggs inseminated quickly in 

 small quantities of sea-water are capable of engulfing sperm. 



The history of the penetration as known may be briefly given. 

 One finds sperm external to the egg at different stages. How it 

 gets into the egg I cannot yet state with certainty although this 

 point has received most careful study for three years. Material 

 has been prepared in every way possible to demonstrate the early 

 penetration. So far I have not found the sperm entering the egg 

 as a slender thread like that in the normal egg. It can be easily 

 demonstrated in the endoplasm. On one slide of the 191 1 series, 

 for instance, I counted twenty sperm heads with their asters 

 lying near the centre of the egg. The sperm head remains for a 

 longer time than in the normal egg a black knot with a long 

 drawn out thread extending to the single aster. A second aster 

 has never been found. The germ nuclei copulate but the eggs 

 never cleave. Various stages are found from sixty to one hundred 

 twenty minutes after insemination — sixty minutes after cleavage 

 in the normal egg. The pronuclei after apposition gradually 

 separate and degenerate as discrete nuclear masses. Many eggs 

 show only one chromatin mass in process of degeneration; 

 doubtless, these are eggs which sperm do not enter. The sections 

 of such eggs closely resemble those of Nereis eggs from which the 

 sperm have been removed (see Lillie, '12). I have repeatedly 

 made observations on living eggs inseminated in sea-water and on 

 sections. I have yet to find a single cleaving egg. 



