ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 397 



water by osmosis several times more rapidly than unfertilized eggs. 

 This phenomenon seemed to him to indicate that the permeability of 

 the egg-surface to water is decidedly increased by fertilization, and 

 hence to confirm the view that a general increase of surface-permeability 

 is an esssential factor in the activation-process. According to Loeb, 

 however, the increased entrance of water after fertilization is due to 

 the removal of the layer of viscous material, or " jelly," which normally 

 invests the unfertilized egg, and disappears at fertilization. 



Lillie's further experiments show (1) that the jelly is variable in 

 freshly-shed eggs and absent in a good proportion ; nevertheless the 

 rate of swelling is remarkably uniform ; the variability in the size of 

 the eggs, after remaining several minutes in the dilute medium, appears 

 no greater than in normal sea-water. (2) The jelly remains in a small 

 proportion of fertilized eggs, yet all such eggs swell at the same rapid 

 rate ; no exceptional slowly-swelling eggs are found. (8) After 

 washing until the jelly is completely removed, in most eggs the effect of 

 fertilization on the rate of swelUng is precisely the same as before. It 

 is clear, therefore, that the increased rate of entrance of water into the 

 egg after fertilization is not due to the removal of an external impeding 

 layer of jelly, but must be referred to a change in the egg itself, and 

 most probably to a change in the properties of the limiting protoplasmic 

 layer or plasma-membrane which controls the osmotic exchange. 

 Experiments show increased permeability. 



If an initial increase of permeability is the critical process of the 

 resting-egg — as apparently also in the analogous process of stimula- 

 tion — it is clear that the condition of increased permeability cannot 

 be permanent ; a reverse change must follow, i.e. a return to the semi- 

 permeable and polarized state of the plasma-membrane, if the egg is 

 to remain alive ; otherwise, diffusion-processes will soon cause the dis- 

 integration of the egg. There has to be a " corrective " surface- 

 alteration. This recovery is to be regarded as the expression of a 

 synthesis and redisposition of the necessary structural materials in the 

 surface-layer. At each cell-division there is this alternation. 



Ccelentera. 



Structure of Metridium.* — G. H. Parker and E. G. Titus describe 

 thirteen fairly well-defined muscles or classes of muscles in this sea- 

 anemone. These represent at least four types of organization : first, 

 independent effectives, as seen in the longitudinal muscles of the acontia ; 

 secondly, simple' receptor-effector systems, like the circular muscles of 

 the column, which, though brought into action by direct stimulation, 

 are probably also under some nervous control ; thirdly, more highly- 

 specialized receptor-effector systems, like the longitudinal muscles of the 

 tentacles, which respond only through nervous stimulation ; and, fourthly, 

 complex receptor-effector systems, as shown in the sense-cells of the 

 column wall in conjunction with the longitudinal muscles of the mesen- 

 teries. These four types of organization may be regarded as representing 



* Journ. Exper. ZooL, xxi. (1916) pp. 433-58 (1 pi.). 



