326 



INVEKTEBEATA 



CHAP. 



from the rest by a horizontal constriction. Sections show two things 

 —first, that this lower sphere contains, besides the vegetative _ cyto- 

 plasm, a certain amount of yolk ; and secondly, that it remains in 

 connection with one of the two spheres produced by the vertical cleav- 

 age, by means of a thin pedicle never completely severed. As tlie 

 cleavage of the two blastomeres from one another becomes complete, 

 the lower sphere coalesces with one of the two upper spheres, and the 

 blastomere so formed is shown afterwards to be CD. The lower 

 sphere has been somewhat inappropriately named- the yolk lobe, for 



A u-.w.s 



Fia, 255. — Vertical sections of the eggs of Dentalium before and after fertilization in 

 order to show the flow of cytoplasmic substances. (After Wilson. ) 



A, liefore fertilization — after extrusion from the oviduct. B, after fertilization — formation of tlie 

 iirst polar spindle. C, division into two blastomeres — extrusion of the first polar lobe, j/.s, germinal 

 spot, i.e. nucleolus ; g.v, germinal vesicle ; l.w.s, lower white substance ; n, the iirst two daughter 

 nuclei separating from each other ; pi, first polar lobe ; u.w.s, upper white substance.' 



which Wilson substitutes the name polar lobe, and its fusion with 

 one of the two blastomeres is known as the retraction of the 

 polar lobe. 



When the next cleavage occurs, AB of course divides into A and 

 B, and the polar lobe is again constricted from CD, but at the con- 

 clusion of the cleavage it fuses with D, which is thus rendered by 

 far the biggest of the first four blastomeres. A and C, moreover, as 

 usual, meet in an upper cross furrow. Each of the four cells contains 

 a portion of the white area which was situated at the animal pole of 

 the egg, but only D has the white material of the vegetative pole. 



