230 



THE CELL 



band connecting them with the main part of the nucleus generally 

 becomes as fine as a hair. Small protuberances develop on the 

 free surface whenever the knob-like nuclear ends touch it. Thus, 

 as these ends grow, each is contained by a special protuberance 

 or cell-bud of its own. The whole cell-bud then increases some- 

 what in size, and becomes constricted at its base from the 

 mother-cell ; the part of the nucleus, which it contains, takes the 

 form of a horse-shoe, separating itself from the delicate connecting 

 thread which united it to the mother- nucleus. The cell-buds are 

 now mature, and after detaching themselves from the mother 

 organism, move about for a time in the sea- water as zoospores. 



II. PARTIAL OR MEROBLASTIC SEGMENTATION. 



If we disregard the case of certain Protozoa (Noctiluca), partial 

 segmentation occurs only in egg-cells. It may conveniently be con- 

 sidered after unequal division. It is found in all cases where the 

 amount of yolk present is extremely great, and where the proto- 

 plasm is clearly separated from it, being collected together in a disc 

 at the animal pole (Fig. 108). The nucleus, which is situated in the 

 centre of this disc, must assume a horizontal position when it de- 

 velops into a spindle. Hence the first division plane is in a ver- 

 tical direction, and appears first at the animal pole in the centre of 

 the disc (Figs. 120 A, 121 A), as in an egg, in which unequal cleav- 



Fio. 120.-Surface view of the first cleavage stage of a Hen's egg (after Coste): o edge 

 tal disc ; b vertical furrow , c small central portion ; & large peripheral portion. 



age occurs (Fig. 92). Whilst, however, it gradually deepens and 

 sinks in until it has cut its way through to the vegetative pole, the 

 germinal disc is divided into two equal segments, which rest like 

 two buds, with their broad bases upon the undivided yolk-mass, 



