242 THE CELL 



At the same time, the furrows already mentioned deepen, so that 

 each spindle comes to lie in a protuberance or bud. Now the 

 splitting up becomes either pretty well completed, or the four 

 spindles, before the furrows have penetrated far into the yolk, 

 commence to divide, the nuclear segments travelling towards the 

 poles. The result of this is that the four first protuberances 

 begin to become constricted a second time and to separate from 

 one another (cell-budding, bud formation). 



The most striking of the phenomena described above is the 

 sudden appearance of the four pole radiations, for which, accord- 

 ing to our present knowledge, an equal number of centrosomes 

 must have served as bases. An explanation of this is afforded us 

 by the processes connected with the fertilisation of the Echinoderm 

 egg, which are discussed on p. 259. 



Modifications of the form of nuclear transformation, shown in 

 Fig. 129 (7, occur not infrequently ; these are due to one of the 

 radiations being somewhat separated from the three others (Fig. 

 130). In this case the three that are situated close to one another 



FIG. 130. 



FIG. 131. 



andTh If: 131 '-* llclear fi S ures th 'our po] e8 from Strongylocentrotu, egg8, which, one 

 ami a half hours after the act of fertilisation, have been piaced for twenty minutes in -05 



?^:-;ir e n so o , f urr e ' and which have ^ kmed tw Jr ^ > 



are united by the three spindles to form a triaster. In the centre 

 I the equilateral triangle thus formed, the three nuclear planes 

 intersect, producing another regular figure. The fourth radiation, 

 which is situated at one side, is connected by a single spindle with 

 the radiation nearest to it. 



w - Fiff ; >i 31 maj be re S arded as an intermediate stage between 

 Figs. 129 and 130. Here the radiation x , which lies somewhat 



