EFFECT OF RADIUM ON FERTILIZATION 93 



In the meantime, the polar bodies have been extruded and the 

 egg nucleus, at first very small after the running together of the 

 vesicular chromosomes remaining after the second maturation 

 division, now enlarges. The chromatin forms at first into kary- 

 omeres, just as in th^case of the sperm nucleus, and then spins 

 out into a delicate spireme. The two germ nuclei now fuse. The 

 spiremes thicken and break up into chromosomes, which, after the 

 first cleavage, become vesicular and finally run together forming 

 the typical resting nucleus of the blastomeres. 



The early cleavage needs no description, but a few typical 

 stages that have been chosen arbitrarily for purposes of compari- 

 son with the abnormal larvae will be mentioned. In ten hours 

 (depending somewhat on the temperature) the embryo acquires 

 the protrochal band of cilia and begins to swim about actively. 

 Green pigment at the posterior end develops in about fifteen to 

 eighteen hours. In thirty five hours reddish brown pigment 

 develops just back of the protrochal ciliary ring. A little later 

 the first setae appear. At sixty hours the setae are in two pairs 

 and soon after become jointed. The third pair also develops at 

 about this time. One hundred hours after fertilization the palps 

 are formed, and the whole embryo is markedly segmented. 



EXPERIMENTAL 

 a. The fertilization of normal eggs by radiated sperrnatozoa 



The spermatozoa of Nereis show remarkable vitality under very 

 adverse conditions. If they are taken from a freshly caught, 

 vigorous male and kept without admixture with sea water, they 

 will remain alive for upwards of fifteen hours, and at the end of 

 that time will fertilize fresh eggs in a normal manner, the embryos 

 arising from such a union showing no marked abnormalities. 

 They can live for a short time in practically fresh water, a fact 

 that renders the sterilization of pipettes a troublesome process. 

 If pure sperm be diluted with sea water the spermatozoa die 

 within a few hours. 



The fresh spermatozoa, as free from sea water as possible, were 

 exposed by putting a small drop of the fluid in a thin walled glass 



