CLEAVAGE AND DIFFERENTIATION 



121 



the polar lobe is at one moment only connected with the rest of the 

 egg by a narrow stalk (figs. 57, 58). 



/ 



Fig. 57 

 Polar lobe formation in the normal cleavage of the Mollusc Ilyanassa. The small 

 circles represent yolk-spheres, a, Protrusion of the lobe by the uncleaved egg. 

 b, c, Its passage to the CD blastomere. d-f, Second cleavage; the polar lobe 

 passes to the D blastomere. g, Formation of first quartet of micromeres by 

 unequal dexiotropic division, h, i, Formation of second quartet of micromeres 

 by an unequal laeotropic division. (From- Morgan, Experimental Embryology, 

 Columbia University Press, 1927, fig. 135, p. 360.) 



Normally, the lobe is composed of very yolky material. Centrifuge 

 experiments, however, show that its protrusion occurs irrespective 



