LARVAL DEVELOPMENT IN THE SEA URCHIN 73 



mensions (fig. 8). The gastrula formed from a blastula like 

 figure 8 is natter, i.e., apical inhibition is greater than in the 

 normal form (fig. 9). Occasionally, in the more extreme cases, 

 the entoderm evaginates instead of invaginating, and an exogas- 

 trula results, but in such cases, so far as my observations go, 

 at least some of the mesenchyme cells pass into the blastocoel. 

 Figure 10 shows the beginning of exogastrulation, and figure 11, 

 an exogastrula. Under no conditions, however, has exogastru- 

 lation been observed in more than one or two per cent of the 

 inhibited forms. 



In the more extreme degrees of inhibition, the form of the 

 body becomes more nearly spherical, the entodermal differen- 

 tiation more completely inhibited and the skeleton more rudi- 

 mentary or completely absent, although, even when they are 

 entirely unable to form a skeleton, the mesenchyme cells are not 

 dead, but persist in the blastocoel. In most cases where a 

 skeleton forms, the angle of divergence between the skeletal rods 

 is narrow. Figure 12 A and B, shows a form with rudimentary 

 narrow-angled skeleton and figure 13, a form in which the skel- 

 etal rods are almost parallel. Figure 14 is an anenteric larva 

 in side view with rudimentary skeleton, a form resulting from 

 an exogastrula; figure 15, an askeletal form with differentiated 

 entoderm, and figure 16, a still more inhibited askeletal form, 

 in which the entoderm never develops beyond the stage of the 

 spherical vesicle attached to the body wall. In forms like figure 

 16, apico-basal and other axial differences in the ectoderm never 

 appear, and the blastopore usually closes completely, so that it 

 is impossible to determine whether the position of the entodermal 

 vesicle represents the basal region or not, and certain cases of 

 differential recovery to be described below will show that the 

 entoderm may entirely lose connection with the blastopore 

 region. In the case of exogastrulae where the entoderm is en- 

 tirely lost, the askeletal forms develop into thin walled ecto- 

 dermal spheres containing scattered mesenchyme cells, and 

 without any visible axial differentiation in any direction. 



In these spherical forms all axial differentiation beyond that 

 of the gastrula is inhibited, and it is of interest to note that in 



