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HANDBOOK OF INVERTEBRATE ZOOLOGY. 



the notch (e), is much deeper, and a small, transparent 

 body, the "direction-cell" (Fig. 45, /), separates from 

 the yolk in the notch. The direction cell takes no part 

 in the development of the embryo, and soon disappears 

 in Arbacia, although, in other animals, it may persist for 

 some time, thus indicating in the embryo the point occu- 

 pied by the principal axis. That end of the principal axis 

 where the direction cell is situated is known as the germi- 

 native pole, while the opposite end is known as the nutri- 

 tive pole. 



The notch deepens rapidly ; soon runs entirely through 

 the egg, and divides it, along the principal axis, into two 

 equal and similar masses, the two primary segmentation 

 spherules (Fig. 45) . At the same time, a circular, slightly 

 transparent spot, the segmentation nu- 

 cleus, becomes indistinctly visible in 

 each spherule. 



FIG. 46. Egg during the period of rest which 

 follows the first period of segmentation. (From a 

 sketch by H. Gannan. ) 

 FIG. 46. 



c. The first division of the egg goes on quite rapidly, 

 but as soon as it is completed, the egg passes into a rest- 

 ing stage ; the two spherules flatten against each other, 

 the fissure between them becomes in- 

 distinct, as shown in Fig. 46, and the 



for some time without 



change. 



FIG. 47. Egg at the beginning of the second 

 period of segmenting activity. 



g. fj. Beginning of the second cleavage furrow. 



FIG. 47. 



d. The next period of activity is initiated by the reap- 



