104 



Mesomeres 

 Macromeres 



Micromeres 



Sensory Cilia 



of 

 Apical Organ 



Invag-inated 

 Enter on 



micromeres 



\ 



Veg.2. 



plus micromeres 



oo 

 oo o 

 ooo 



Exogastrula 



Fig. 47 

 Diagram illustrating the developmental potencies of isolated fragments of the 

 sea-urchin embryo, representing different levels along the egg-axis. Note distri- 

 bution from animal to vegetative pole of potencies for the formation of apical 

 organ, ectoderm with cilia, and invaginated endoderm. The position of the 

 third (equatorial) cleavage plane varies; when nearer to the animal pole, an 

 isolated veg. i disc has more animal and less vegetative potencies, which it 

 shows by forming an apical organ and not forming a gut ; when the cleavage plane 

 is nearer to the vegetative pole, an isolated veg. I disc invaginates a small gut 

 and forms no apical organ. (Original, based on Horstadius.) 



