72 C. M. CHILD 



'normal form) shows that in the former the oral lobe is shorter 

 and less developed, the anal arms slightly shorter and the angles 

 of divergence between the lateral skeletal rods and between oral 

 lobe and arms are slightly smaller than in the latter, while the 

 length of the body from the base of the anal arm is essentially 

 the same. In these plutei the development of the oral lobe, 

 which represents the apical region, is most inhibited and the 

 anterior end is smaller in both dimensions than in the normal 

 animal, while the posterior region is fully developed. Forms 

 such as this, in which the angles of divergence of the skeletal 

 rods are less than the normal, are called, for convenience, nar- 

 row-angled forms. These differences may appear in different 

 degree in different individuals of the same lot, in some the only 

 difference from the normal being a slightly shorter oral lobe, 

 while in others, where the oral lobe is inhibited to a greater 

 degree, the angles of divergence, which are indices of the 

 development of the anterior end in the direction of the minor 

 axes, are also smaller than in the norm. Where the degree of 

 differential inhibition is so slight, the earlier development is 

 merely somewhat retarded without appreciable departure in its 

 course from the norm, and the differential inhibition begins to 

 appear only in the later stages. 



A somewhat greater degree of inhibition produces forms 

 which attain the condition of figure 7 A and B, but develop no 

 further. Here the larva remains small, the development of the 

 oral lobe is completely inhibited, the anal arms remain short, 

 and the dimensions of the anterior end, as compared with more 

 posterior regions, are much reduced, so that the angle of diver- 

 gence between the skeletal rods is also reduced as compared with 

 the normal form. The entoderm, however, still undergoes 

 differentiation into three parts and the mouth forms. It is evi- 

 dent that in such cases the development of the apical region is 

 most inhibited and the development of the anterior end more 

 than that of the posterior end. 



Where the degree of inhibition is still greater, the differential 

 inhibiiton may appear even as early as the elongated blastula 

 stage, as a relative decrease in apical and increase in basal di- 



