THE VITAL PHENOMENA OF THE CELE 



231 



and are thus connected with one another. Soon afterwards a 

 second vertical furrow makes its appearance, crossing the first at 

 right angles, and terminating in a similar manner at the germinal 

 disc, which is now split up into four segments (Figs. 120 B, 121 B). 

 In this case also a cleavage line is formed. 



B 



G 





Fig. 121. Discoidal cleavage of the egg of a Ccphalopod (after Watase; from Hertwig, 

 Fig. 99). 



Each of the four segments is again halved by a radial furrow. 

 The segments so produced correspond to sectors, whose pointed 

 ends meet in the centre of the germinal disc, and whose broad 

 ends are turned towards the periphery. The pointed ends are 

 separated from the rest of the segment by a diagonal furrow, or 

 by one which is parallel to the equator of the egg-sphere ; and 

 in consequence smaller central segments cut off from the yolk 

 in every direction, and larger peripheral portions still connected 

 with the yolk, may be distinguished (Fig. 120 C). From now on, 

 furrows which are radial, and ones which are parallel to the equa- 

 tor, alternately make their appearance, so that the germinal disc 

 becomes more and more split up, the segments being so arranged 

 that the smaller ones are in the centre of the disc, and the larger 

 ones on its circumference (Fig. 121 G). Many of the segments 

 which are still attached to the yolk become constricted, so that the 

 nuclear spindle is slanting or vertical, the consequence of which is, 

 that when division occurs one of the daughter nuclei is situated 

 in the yolk-mass. In this manner the yolk-nuclei are produced by 

 partial cleavage ; an especially large number of them are em- 

 bedded in the superficial layers of yolk on the periphery of the 

 segmented germinal disc. Compare the interesting observations 

 of Riickert (VII. 36), and Oppel (VII. 34), from which it appears 

 that in Selachians and Reptiles yolk-nuclei develop in consequence 

 of over-impregnation. 



