EMBRYOLOGY OF ECHINODERMS. 



105 



pearance of the distinct furrow between the two spherules. 

 The segmentation nuclei then become invisible (Fig. 47), 

 and traces of a second division make their appearance at 

 right angles to the first, but, like the first, in the plane of 

 the principal axis. Four segmentation nuclei now appear 

 in place of the two, and the egg soon becomes divided 

 into four spherules, as shown in Figs. 48 and 49. The 

 first of these figures gives a polar view, or a view in the 

 line of the principal axis, while the second is a side view, 

 or one at right angles to this axis. 



FIG. 4s. 



FIG. 49. 



FIG. 50. 



FIG. 48. Egg at the end of the second period of activity, viewed 

 from one end of the principal axis. ( From a sketch by H. Garman. ) 



FIG. 49. The same egg viewed at right angles to the principal axis. 

 (From a sketch by H. Garman. ) 



FIG. 50. An egg during the resting stage which follows the second 

 period of activity, seen from one of the poles of the principal axis. 

 (From a sketch by H. Garman.) 



e. The five spherules now flatten against each other, the 

 line between them becomes indistinct, and the egg passes 

 into the resting stage (Fig. 50). 



f. The spherules again become distinct, and a plane of 

 division makes its appearance at right angles to the prin- 

 cipal axis, and soon divides each of the four into two, so 

 that the egg now consists of eight (Fig. 51). 



(j. This division is followed by a resting stage, shown 

 in Fig. 52. 



