74 C. M. CHILD 



these cases the mesenchyme cells are scattered irregularly and 

 do not aggregate in the basal region, as they do to a greater or 

 less extent in less inhibited or normal forms. Evidently the 

 localized differences in the ectoderm which supposedly deter- 

 mine the distribution of the mesenchyme are not present to a 

 sufficient degree to be effective. 



The most extreme degrees of differential inhibition are partial 

 forms in which a portion of the body has been killed. In these 

 cases death proceeds from the apical region, and the part which 

 remains alive therefore consists of more or less of the basal 

 region (Child '16 c). When such partial apical death occurs be- 

 fore the gastrula stage, the gastrulae are small, with a dispropor- 

 tionately large entoderm, though not always as large as the nor- 

 mal (figs. 17 and 18), gastrulation itself apparently being inhib- 

 ited to some degree. When partial apical death occurs after 

 gastrulation, the result is not essentially different. These partial 

 basal forms show no ectodermal differentiations, remain spheri- 

 cal, the blastopore closes, and the entoderm remains as a spheri- 

 cal vesicle (figs. 19, 20, 21). As the figures indicate, they vary in 

 size according as they represent a larger or smaller fraction of 

 the apico-basal axis, and in the smaller forms, such as figures 20 

 and 21, the entodermal vesicle completely fills the blastocoel, 

 because the whole of the entoderm, and only the basal portion 

 of the ectoderm, remain alive. 



In all these spherical forms, whether whole (fig. 15) or partial 

 (figs. 19 to 21), the entoderm often loses its coherent epithelial 

 character after several days of life without further development. 

 In figure 22 this process is indicated. The cells become scat- 

 tered about the blastocoel, and such forms are distinguishable 

 from the anenteric forms resulting from exogastrulation, only 

 by the larger amount of cellular material in the blastocoel. I 

 am inclined to regard this disintegration of the entoderm as con- 

 nected with its lack of functional activity, and it may be that 

 obliteration of the metabolic gradient in the entoderm is also 

 concerned. At any rate, this entodermal disintegration has been 

 observed only in the inhibited forms, where the entoderm is cut 

 off from the basal region and does not differentiate. 



