86 C. M. CHILD 



or the basal ring (figs. 63 to 64), the apical outgrowth (figs. 65 

 to 72), or both (fig. 73) may be absent. It is of interest to note 

 that those forms with well developed basal ciliated ring move 

 with basal end in advance almost as frequently as in the opposite 

 direction. 



In some of these forms the entoderm shows normal relations 

 and undergoes normal differentiation (figs. 57, 65, 66), but more 

 usually the blastopore closes completely, and the entoderm loses 

 connection with the basal region, but remains attached to the 

 apical region. Under these conditions a mouth may develop 

 and differentiation of the entoderm into the three parts may 

 occur (figs. 63, 67 to 69) ; and in such cases the rectal region 

 either ends blindly (figs. 63, 68, 69), or occasionally a new anal 

 opening arises, apparently wherever the rectal region is in con- 

 tact with the body wall (fig. 67). In other cases mouth, oesopha- 

 gus, and stomach-intestine are present, but the rectal region fails 

 to develop or, perhaps, becomes separated from other parts and 

 degenerates (fig. 61), as in some cases of acclimation (see p. 81). 

 In still other cases mouth and oesophagus may be absent; but 

 stomach-intestine and rectal region may differentiate (figs. 58, 

 59, 73), or the entoderm may show little or no trace of regional 

 differentiation (figs. 60, 64, 70, 72). Occasionally the entoderm 

 remains in contact with the blastopore region, though com- 

 pletely closed and without regional differentiation (fig. 64), and 

 a few individuals with evaginated entoderm (figs. 62, 71) re- 

 sulting from exogastrulation (p. 73) occur. 



These various degrees of development and relations of the 

 entoderm depend on the degree of inhibition, the time when 

 apico-basal elongation begins and perhaps on other factors as 

 well. In the more extreme degrees of inhibition the blastopore 

 usually closes completely and the entoderm becomes a closed 

 vesicle. If acclimation or recovery occurs in such cases the 

 entoderm usually becomes attached to the apical region of the 

 body wall, and its further development depends on conditions 

 in this region of the body wall and on the degree of entodermal 

 inhibition. In some cases, where the blastopore does not com- 

 pletely close, the entoderm may reach the apical region and retain 



