232 PATTERNS AND PROBLEMS OF DEVELOPMENT 



inhibited, also gives off cells (Fig. gs, A, C, D, I). Thus far, little differ- 

 entiation has been seen in the remaining ectoderm of starfish exogastrulae, 

 even in recovery. In forms with considerable entodermization the remain- 

 ing ectoderm represents more or less of the prospective preoral region, 

 but there is no evidence that it reconstitutes a more or less complete 

 larval ectoderm. It may become rounded and in some individuals an 

 apical thickening appears as a secondary modification (Fig. 95, E, F). 

 Extensive, perhaps complete, entodermization of ectoderm and loss of 

 epithelial character in the prospective entoderm results in forms like Fig- 

 ure 93, G, of Dendr aster, an effect of long-continued exposure in both; 

 and more or less entodermal dissociation may occur in later gastrula 

 stages without entodermization or exogastrulation (see Fig. 85). Whether 

 dissociated cells of the prospective entoderm may function as mesenchyme 

 is not known; but here, as in echinoids, they appear to be merely in- 

 hibited cells without particular function. 



In the asteroid, as in echinoids, lithium entodermizes prospective ecto- 

 derm and, after the increase in entodermal susceptibility occurs, also in- 

 hibits entoderm. Exogastrulation has also been produced in Patiria by 

 crowding and by acidified sea water, by KCN, and by Janus green; in 

 Asterias by lack of oxygen. With all these agents there is apparently more 

 or less entodermization of prospective ectoderm. Probably still other 

 agents will produce exogastrulation and entodermization in starfish, as in 

 echinoids. 



EXOGASTRULAE AND PSEUDO-EXOGASTRULAE 



In this discussion of exogastrulation all forms with external entoderm 

 have been called "exogastrulae," but they are not all alike in origin. 

 Strictly speaking, exogastrulae are modifications in which the entoderm 

 evaginates instead of invaginating. In many so-called "exogastrulae," 

 however, the original prospective entoderm invaginates, and the external 

 entoderm is entodermized ectoderm (Figs. 90, D, and gi, D). These have 

 been called "entexogastrulae," or, when the invagination is secondary, 

 "exentogastrulae" (Fig. 95, H, I). In many cases, also, the prospective 

 entoderm takes little, perhaps no part, in formation of the external ento- 

 derm but loses epithelial character and remains a cell mass, usually with 

 more or less complete dissociation, while the external entodermal epithe- 

 lium is, in large part or wholly, entodermized ectoderm, as in Figures 91, 

 B, C, E, G, and I, and probably also in F and H. The entodermized ecto- 

 derm of these forms has not evaginated instead of invaginating, and it 

 shows no tendency to invaginate after return to water. If evagination re- 



