STRUCTURE OF PROTOPLASM. 315 



The wall immediately after fertilization shows no increase 

 in thickness and is still very pliable, extensile, and contractile. 

 A little later it stiffens up until no longer elastic, though slightly 

 resilient and pliable in so far that permanent indentations can be 

 made. It can not now be severed by two approaching needles. 

 In those few instances where the tough wall was punctured, and 

 the protoplasm forced out, the ejected mass immediately devel- 

 oped a membrane. This plasma membrane is exceedingly thin, 

 and quite elastic. It is interesting to note that the freed pro- 

 toplasm here forms a delicate pellicle- not the tough wall of the 

 one-celled embryo from which it came, nor the substantial though 

 glutinous egg-wall which it would have developed in repairing a 

 tear before fertilization. 



At all times, from the moment of fertilization until dissection 

 is no longer possible, the embryonic protoplasm is non-miscible. 

 The ejected plasma shows no tendency to mix with the surround- 

 ing sea-water until after gelating, i. e., after death. 



The Midticellular Embryo. 



As the two-celled embryo stage is neared the former egg 

 wall has lost all of its former elasticity and glutinosity. It 

 behaves under pressure like a ball of thin celluloid, resisting light 

 pressure and returning to its former shape, unless forced beyond 

 a certain point, when depressions made are retained. The two- 

 celled embryo is punctured with great difficulty. The wall of 

 the four-celled embryo is a very substantial affair, two microns 

 in thickness, highly resilient and impenetrable to the sharpest 



needle. 



The Degenerate Egg. 



The behavior of the over-ripe ovum is extremely interesting 

 and instructive in its bearing on the mechanism of the egg con- 

 tents. As already stated, an ovum ultimately gelates and usually 

 disintegrates. A gelated egg may be torn apart with great ease. 

 If hardening has progressed far enough the egg may be cut in 

 pieces as one would cut butter. If, on the other hand, the con- 

 tents are not too solid the matrix may be readily drawn out into 

 a long invisible thread. It is this gelated cytoplasmic matrix 

 which is water-miscible. 



