MUSEUM OF COMPARATIVE ZOOLOGY. 179 



Up to this time the egg has remained without perceptible change of 

 outline. With continued increase in the extent of the cap of clear 

 protoplasm, which in section appears crescent-shaped, there is a slight 

 elongation of one axis of the yolk, which gradually becomes more notice- 

 able in the form of a low conical elevation at the side already indicated 

 as that toward which the central spots tend. For the sake of precision 

 I will call this the animal pole of the yolk, the opposite, the vegetative 

 pole. 



Thus far the changes, whether within or without, have been so slow 

 as to be recognizable only after the lapse of some minutes ; but now 

 there begins at the middle of this crescent-like thickening a more rapid 

 movement. The centre of this clear portion of protoplasm rises promptly 

 in the form of a low, rounded eminence, of limited extent, which first 

 becomes somewhat conical, and then assumes a more rectangular out- 

 line, in that its sides become nearly parallel. In this condition, it is 

 really a low cylinder, with one end free and rounded, the other in con- 

 tinuation with the vitellus. (Fig. 1.) Sometimes this elevation seems to 

 remain almost entirely free from opaque substance ; at other times, gran- 

 ules accumulate to such an extent as to make the central portion of the 

 protuberance appear very dark in transmitted light, and correspondingly 

 white when seen by reflected light. The outline of the protuberance is 

 sometimes slightly irregular and angular, although usually it is quite full 

 and rounded. Without cessation it continues to change, principally by 

 the mutual approximation of the sides of the cylinder at its base. This 

 approximation takes the form of a constriction which is at first (Fig. 2) 

 a broad furrow extending all around the cylinder. This furrow grad- 

 ually becomes narrower and deeper (Fig. 3), and the excrescence which 

 is thus being cut off takes a distinctly rounded form. The granulations 

 now often appear gathered into the distal portion of the protuberance. 

 Finally, the constriction deepens until there is only a slender thread of 

 protoplasm joining the smaller and the greater sphere. This often per- 

 sists for some time (Fig. 15), but finally ruptures, and sets free a small 

 spheroidal body, with perfectly sharp and delicate outline, which is the 

 first " polar globule." It is only five or ten minutes from its first ap- 

 pearance till it has the form of a sphere attached by a slender thread. 



During the formation of this first polar globule other changes are 

 taking place, to the consideration of which it is now necessary to return. 

 The elongation of the yolk in the direction of the animal radius (as I 

 shall call that radius which terminates in the animal pole) is very soon 

 followed by its flattening in the direction of the same line. It is the 



