466 THEOPHILUS S. PAINTER 



droxide, so I left Naples without reaching any conclusion on this 

 point. Unknown to me then, Goldschmidt and Popoff ('08) 

 had carried out a series of experiments, on sea urchin eggs, bear- 

 ing on this general question, and their work sets the matter in 

 such a clear light that I will review their paper in some detail. 



In the ctenophores, the swelling of the ectoplasmic layer is 

 very conspicuous, and Ziegler ('97) had assigned to it a con- 

 stricting action in the cleavage of these eggs. This lead Gold- 

 schmidt and Popoff to investigate the origin of this layer, and 

 the conditions under which it appears. They found that the 

 layer appeared about the time the pronuclei of the egg were 

 fusing and it steadily increased in thickness as the nucleus pre- 

 pared for division. Just before cleavage set in, a girdle of 

 swollen ectoplasm marked out the cleavage path. 



When the water was allowed to evaporate from the dish in 

 which the eggs were placed, the membrane grew thicker and 

 this lead the authors to suspect that osmotic changes were 

 responsible for its appearance. When eggs were placed in 

 hypertonic sea water as the spindle was being formed, the layer 

 greatly increased in thickness, at the same time numberless 

 little processes of granular cytoplasm penetrated into the trans- 

 parent layer, anastomosing and branching, giving the layer a 

 vacuolated appearance. As a rule such eggs did not divide, 

 but when a cleavage furrow did appear, the ectoplasmic layer 

 did not sink in with it as it does under normal circumstances. 

 In the space thus left, little radiating fibers (Zuglinien) appeared 

 running from the granular cytoplasm to the layer. Eggs treated 

 with hypertonic sea water showed only an extremely thin layer 

 of ectoplasm in later stages. Also the authors reached the 

 conclusions that the ectoplasmic layer, in later stages, is a true 

 membrane and not an integral part of the egg. 



These experiments show that osmotic changes cause the 

 swelling of the ectoplasmic layer, in fact, that an increase in 

 the density of the sea water causes the increase in thickness, 

 while hypotonic sea water tends to inhibit its appearance. 



Returning now to the monaster eggs, since the density of the 

 sea water remains the same, we seem forced to conclude that 



