MESOMERES 

 MACROMERES 



SENSORY CILIA 



OF 

 APICAL ORGAN 



VEGETATIVE II 

 PLUS MICROMERES 



EXOGASTRULA 



Fig. 163. Developmental potencies (cell lineage) of isolated blastomeres of the cleav- 

 ing sea-urchin egg. representing different levels along the egg axis (from Huxley and 

 DeBeer, '34, after Horstadius). Observe the following: (1) Progressing from the animal 

 pole to the vegetative pole, the potency for developing the sensory cilia decreases from 

 animal pole cells I to animal pole cells II. (2) The potency for developing motile cilia 

 increases from animal pole cell II to vegetative pole cell I. (3) The potency for gastrula- 

 tion becomes greater from vegetative pole cell I to vegetative pole cell II. (4) In the 

 development of vegetative pole cell I, shown at the right of vegetative I, if the third 

 (equatorial) cleavage plane happens to be displaced near the animal pole, an isolated 

 vegetative cell I has more animal pole potencies and will develop apical cilia; if the 

 cleavage plane is displaced toward the vegetative pole, the vegetative pole cell I will 

 attempt to gastrulate. (5) The disc of vegetative cells II plus the micromeres produce a 

 gut so large it will not invaginate and hence forms an exogastrula. 



329 



