114 CHARLES PACKARD 



sions. Development is consequently parthenogenetic, a condition 

 which does not obtain in the Nereis egg. It should be noted 

 that the sea urchin egg is an especially favorable subject for par- 

 thenogenetic development, while Nereis is not. Consequently 

 the two cases are not exactly parallel and the behavior of the 

 one should hardly be expected to be repeated in the other. 



The generally accepted theory that the cleavage centrosomes 

 are derived from the divided centrosomes brought in with the 

 sperm has recently been questioned by Lillie who believes 

 that in Nereis the centrosome appearing at the base of the sperm 

 head is derived through the interaction of the sperm chrom- 

 atin and the egg protoplasm. ' The fact that the middle piece 

 is always left behind on the outside of the egg membrane seems 

 to support this view. A discussion of the theory is not in 

 place here, but it may be pointed out in passing that this fact 

 alone is not conclusive evidence that no formed centrosome has 

 been introduced by the sperm, since in many insect sperma- 

 tozoa the centrosome is contained in the sperm head. If, 

 however, in Nereis, the centrosome is a product of the sperm 

 chromatin and the egg protoplasm we have evidence that the 

 sperm nucleus has been injured. If, on the other hand, the 

 older view as to its origin is true we have evidence that the 

 radium radiations have affected a protoplasmic structure. 



A discussion of the effect of radiations on the unfertilized egg 

 raises some interesting questions concerning certain protoplas- 

 mic activities of the egg. It has been shown that in many 

 cases the cortical zone, composed largely of colloidal material 

 is not extruded at the time of implantation of the sperm, but 

 remains for a considerable period within the egg, and is given 

 off slowly during subsequent development before cleavage. As 

 a result polyspermy occurs. The extrusion of the surface layer is 

 due, according to R. S. Lillie, to the sudden increase of per- 

 meability of the plasma membrane, resulting in an outflow of 

 substances previously impermeable to it. That the membrane 

 is rendered somewhat permeable at the time of implantation of 

 the sperm is evident from the fact that water is taken up, result- 

 ing in a swelling of the alveoli. But the normal permeability is 



