276 OTTO GLASER. 



tion of them. Material leaves the egg and the egg shrinks. 

 These two processes appeared to me as intimately linked and for 

 part of the time simultaneous. Furthermore I discussed the 

 material which the egg eliminates ('14, p. 91). This secretion or 

 excretion, as Loeb ('08) long ago rendered probable, contains a 

 colloid to which the fertilization membrane itself is impermeable. 

 The substance in question completes the conditions under which 

 the perivitelline space can exert an osmotic pressure greater than 

 that of the sea-water outside. Accordingly the membrane must 

 become distended and its actual distension is so clear optically 

 that no one before has ever doubted it. 



Summarizing, we may say: the egg shrinks away from its 

 enclosing membrane and the membrane becomes distended. 

 These two movements in opposite directions account for the 

 existence and volume of the perivitelline space. 



III. 



But does the egg actually shrink? Chambers denies the fact. 

 The Asterias egg, he claims, gives no indication of volumetric 

 decrease. If one waits long enough there is a measurable 

 increase which I failed to notice because my "measurements 

 were made on the assumption that the eggs always maintain a 

 spherical shape." However the Asterias ovum is "very soft and 

 if allowed to lie on the bottom of a glass dish tends to flatten into 

 the shape of a disc," which "upon fertilization . . . rounds up as 

 the fertilization membrane leaves its surface" (loc. cit., . . ., 

 p. 332). Under these conditions, observations taken in one 

 plane would inevitably lead to erroneous conclusions a deduc- 

 tion which is logically incontestible ; which applies without 

 question to the starfish egg at certain times and yet whose 

 inapplicability to my work on Arbacia can be expressed quantita- 

 tively as 98.5 per cent. 



For the sake of the discussion, I will assume that the eggs 

 with which I worked were somewhat distorted by weight. It 

 nevertheless remains true that in Arbacia gravity and surface 

 forces quickly establish an equilibrium in which the unfertilized 

 egg can maintain itself without appreciable change, for at least 

 four hours ('14). During this time further movement toward or 

 away from the spherical condition is negligible. 



