5IO 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



because ectodermal development does not proceed far. Reconstitution of 

 ectoderm is difficult to account for in terms of the Runnstrom hypothesis, 

 but in terms of a quantitative primary gradient it is again essentially 

 similar to what happens in isolated pieces from the lower levels of hydroid 

 stems and planarian bodies. 



Isolation of the whole presumptive ectoderm {an^, an^, veg„ Fig. 145) 

 results in forms either wholly ectodermal, with ciliated band surrounding 

 an oral region, a stomodeum, but no oral lobe or arms and no skeleton 

 (Fig. 164, A, B), or with small entoderm and rudimentary skeleton (Fig. 

 164, C, D). These isolated parts represent approximately three-fourths 



Fig. 164, A-D. — Range of forms resulting from reconstitution of whole presumptive ecto- 

 derm (ani, anz, vegi) of Paracentrotus. A , B, with large scale of organization along primary 

 gradient; C, D, with smaller scale of organization (after Horstadius, 1935). 



of the polar axis; consequently, concentration of vegetal substance should 

 be considerable in their basal regions, but many of them develop no trace 

 of entoderm. Moreover, veg-, alone develops entoderm more frequently 

 and may approach the normal pluteus form, as shown above. How is the 

 very different development of the same axial level in the two cases to be 

 accounted for? According to Horstadius, there is lack of balance between 

 the two gradients in the isolated whole ectoderm, and the animal gradient 

 may suppress the vegetal, while in isolated vegi the two are more nearly 

 balanced. In terms of metabolic gradients the ectodermal gradient and 

 dominance may persist in the apical three-fourths because with good 

 physiological and external conditions there is nothing to determine ento- 

 dermal levels. With less vigorous animals or with slightly inhibiting con- 



