92 CHARLES PACKARD 



which time the jelly loses its sticky character. The eggs were 

 killed in Meves' modification of Flemming's fluid. The sections 

 were cut 5 n in thickness and stained with iron hematoxylin. 



THE NORMAL DEVELOPMENT OF NEREIS 



The normal fertilization of Nereis has been described by Lil- 

 lie ('12) so that an extended account here is unnecessary. My 

 observations are in every respect in accord with his. The follow- 

 ing is a greatly abridged history of the fertilization phenomena in 

 which only the more important events are mentioned. 



As soon as the successful spermatozoon is implanted in the egg 

 membrane it loses its motility and remains exterior to the egg for 

 about fifty minutes. Very shortly after its implantation the 

 cortical zone of colloidal material in the egg begins to be extruded, 

 forming a thick jelly which, as it flows out, pushes away all the 

 spermatozoa except the implanted one. The surface of the egg 

 just beneath the perforatorium of the sperm now rises up in the 

 form of a cone (fig. 1). The fertilization cone thus formed soon* 

 sinks, drawing with it the sperm which consequently comes to lie 

 in a depression of the vitelline membrane. The cone, in the 

 meantime has taken a deep stain and is a conspicuous object at 

 the periphery of the egg. About fifty minutes after insemination 

 the cone begins to move inwardly toward the center of the egg 

 pulling with it the sperm head, which, as a consequence of being 

 drawn through the membrane, assumes a band-like appearance. 

 (fig. 2). The middle piece and the tail are left behind. The 

 cone, followed by the sperm head, now moves deeper into the egg 

 and revolves through 180° so that the sperm head is in front. The 

 cone is left behind at this stage while the sperm nucleus moves 

 forward. At this time a sperm aster develops (fig. 3). Later 

 this structure divides unequallj^, the two asters thus arising form- 

 ing the cleavage asters. As the sperm head advances the chrom- 

 atin becomes vacuolated and later breaks up into the haploid 

 number of karyomeres. These gradually fuse, the chromatin 

 spinning out into a delicate spireme. At this time the sperm 

 nucleus is ready to fuse with the egg nucleus (fig. 4). 



