214 



INTERNAL FACTORS 



IV. i 



cells of this quartette (4 a 4 c) become with the residual macro- 

 meres (A D) the endoderm (or mesendoderm). 



A peculiar structure seen in some forms (Ilyanassa, Denfah'nm, 



FIG. 121. --Normal cleavage of Ilyanassa. A, B. Formation of polar- 

 lobe or yolk-lobe (r77) ; C. l Trefoil ' stage ; D, E, F. The polar-lobe 

 passes into CD ; G. The second polar-lobe protruded in H ; it passes into 

 D. (After Crampton, from Korschelt and Heider.) 



Chaetopterns, Myzosloma] is the yolk-lobe, or polar lobe, as it should 

 properly be called. This is a mass of cytoplasm protruded at the 



