III. i GRAVITATION 79 



is placed mainly on one side, while on the other the lighter 

 protoplasm is more abundant, the yolk granules far smaller and 

 more sparse. The distribution of these two substances determines 

 indeed the axis about which the egg has a ' rotation structure ' 

 or is radially symmetrical. The symmetry is further marked by 

 the disposition of the pigment and the position of the nucleus. 

 The pigment is placed in a thick superficial layer in the proto- 

 plasmic portion, it extends over rather more, sometimes con- 

 siderably more, than a hemisphere, for there is much variation 

 in this respect, and its boundary is a circle whose plane is at 

 right angles to the egg-axis the line which passes through the 

 centre of the egg, the centre of the pigmented portion or animal 

 pole and the centre of the unpigmented portion or vegetative 

 pole. There is also an axial, less-deeply pigmented plug in the 

 animal hemisphere. The nucleus is placed axially, but excentri- 

 cally, very much nearer the animal than the vegetative pole, in 

 a pigment-free spot or ' fovea germinativa '. 



The egg is invested with a layer of jelly (mucin), inside which 

 it becomes eventually free to rotate. This, however, is not possible 

 when the egg is first laid, for the jelly is at that time closely 

 adherent to it. In water, however, the jelly swells up, and 

 a narrow cavity is formed in about three hours between it and 

 the egg, and the egg then turns over until its axis is vertical. 

 The formation of the cavity is much more rapid if fertilization 

 (insemination) has taken place ; in this case the egg turns over 

 in half an hour. The rapid formation of the perivitelline fluid 

 is the first effect of insemination, and is due to some substance 

 secreted by or accompanying the sperm, since the spermatozoon 

 does not reach the egg for another quarter of an hour (O. 

 Hertwig). A second effect is an alteration in the viscidity or 

 cohesion of the egg-contents ; for while in the ovarian or uterine 

 egg no alteration occurs apparently in the disposition of yolk, 

 cytoplasm, and pigment, although the egg-axis may make any 

 angle with the vertical, such an alteration is undoubtedly pro- 

 duced by gravitation (see below) after fertilization has occurred. 

 Another effect noted by Roux is that fertilized (not merely 

 inseminated) eggs turn over more rapidly in a medium of like 

 specific gravity than do unfertilized. 



