IV. i INITIAL STRUCTURE OF THE GERM 207 



of the eight-celled stage. This is attributed by Zeleny to the 

 greater time and opportunity afforded them of regaining the 

 polarity of the whole. 



The behaviour of egg-fragments depends not merely on the 

 part of the egg from which they are taken but also on the time 

 at which they are removed. 



Pieces taken from the ovum, no matter in what way, before 

 the disappearance of the germinal vesicle segment as wholes and 

 give rise to perfect Pilidia. Only in a small percentage (15 %) of 

 cases is their development defective, the lappets being absent, or 

 the gut abnormal, or the apical organ absent or multiplied (Yatsu). 

 The proportion of perfect Pilidia obtained from fragments 

 removed a little later, at the time of 

 formation of the first polar spindle, is, 

 however, less, only 52 %. / /^^.. ^^^x 



It is during the conjugation of the 

 pronuclei that the definite localization 

 of specific substances seems to begin, 

 for now the fate of the fragment de- 

 pends upon the direction in which the 



cut is made. Eggs from which the animal 



,. , ,,. , ,. ,, , , FIG. m.Cerebrettttlua. 



portion about one-third ot the whole Di agram O f egg with one 



has been taken away become perfect polar body, and basal pro- 



TVVV 11 i, i e tuberance. to show direc- 



Pihdia; when, however, by removal of tions of horizontal [H)t 



the vegetative third, or by an oblique vertical (V), and oblique 



cut, the fragment does not contain ( 1 $ )4 c ) uts> (Mter Zeleny> 



the whole of the lower two-thirds, the 



lappets are defective and the apical organ sometimes absent as 



well. If, therefore, there is present at this stage a special 



material for this organ it cannot as yet be located in its ultimate 



position. 



The cleavage of these fragments is always partial (Zeleny). 

 After the first furrow the material for the apical organ has still 

 not reached the animal pole : the animal region may be removed 

 from both blastomeres without inhibiting its formation. Perfect 

 Pilidia may' also be produced from eggs in the two-celled stage 

 when part of one blastomere has been obliquely cut away. 

 Should, however, the blastomeres be cut apart before their complete 



