178 BULLETIN OF THE 



promptly swell when, by rupture of the yolk, they are brought into 

 contact with water, and assume spherical contours, with delicate out- 

 lines and diameters varying from 2 to 8 /x. Others remain small and 

 of greater refractive power, under the same circumstances. They sel- 

 dom exceed a fraction of a micro-millimeter (/x) in diameter. (Fig. 26.) 

 These granules are held in suspension by a viscid transparent proto- 

 plasm. Their distribution is not always uniform, so that an irregular 

 cloudy appearance often characterizes the yolk. In any optical section 

 the peripheral portions of the sphere, owing to the diminished quantity 

 of these granules which the light is compelled to traverse, seem less 

 opaque than the central portions. A very thin shell of protojDlasm 

 at the surface is entirely destitute of granulations, though the yolk is 

 certainly not provided with a distinct membrane, the so-called memhrana 

 vitellina. Toward the centre of the sphere the opacity is not, however, 

 a constantly increasing one, for at or near this point there appears an 

 elongated lighter portion, which is not distinctly limited, but shades 

 gradually into the darker surrounding portions. This is caused by the 

 absence of yolk granules from the central part of the vitellus. I have 

 not been fortunate enough to secure an egg in which this central spot 

 was perfectly spherical, as did Warneck; already a lengthening had 

 taken place, and in most cases it appeared as two contiguous lumi- 

 nous areas. These become more extensive in the course of a few min- 

 utes, and soon appear so displaced that one is much nearer the surface 

 of the yolk than at first. It is only in exceptional cases that these 

 light spots can be seen, previous to the time when one of them appears 

 near the surface. The more superficial spot is then the more conspic- 

 uous. As these draw nearer to one side of the yolk, the granules seem 

 gradually to recede from that side toward which the clear bodies are 

 tending, so that a considerable portion of the yolk appears comparatively 

 transparent. After several minutes the outer spot reaches the surface, 

 and is less sharply marked, probably because of the increasing transpar- 

 ency in the surrounding substance. The deeper spot is now very near 

 the centre of the yolk, and only faintly indicated. After a short time 

 the outer spot is flattened against the surface, and gradually acquires a 

 greater superficial extent. There is now a slow accumulation of per- 

 fectly clear protoplasm at this side of the yolk ; it is thickest where the 

 light spot first touched the surface, and thins away gradually on all 

 sides. This is all accomplished in about an hour after extrusion, though 

 liable to some variation, the changes being more rapid in proportion to 

 the elevation of temperature. 



