236 PATTERNS AND PROBLEMS OF DEVELOPMENT 



mined, we might hope to answer this question; but, although various 

 hypotheses have been advanced in terms of mechanical factors, differen- 

 tial growth, specific constitution of the basal region, differential colloidal 

 swelling inside and outside, etc., they remain hypotheses until we know 

 more about living protoplasms and how external agents act on them. 

 The hypothesis of differences in colloidal swelling as the determining fac- 

 tor not only in gastrulation but in other invaginations, and of its reversal 

 by agents producing exogastrulation, was advanced by Spek (1918). 

 Evagination of prospective entoderm suggests reversal of polarity of some 

 sort in the cells of this region. Differential dye reduction indicates lower 

 oxygen content in the blastocoel than outside (see pp. 133-40). Conceiv- 

 ably, this differential or some other between blastocoel and exterior may 

 induce gastrulation, either by colloidal changes or otherwise, and the 

 activation of the prospective entoderm preceding gastrulation in echinoids 

 may be expected to make it more susceptible to such a differential; but 

 in the starfish the activation is apparently less marked before gastrulation 

 than in echinoids. Perhaps the agents which bring about evagination ob- 

 literate or reverse a polarity in the entodermal cells. According to Lindahl 

 (1936), lithium inhibits one component of respiration in sea-urchin em- 

 bryos. A point of some interest is that the stomodeum, instead of invagi- 

 nating, often evaginates and is considerably enlarged in exogastrulae, sug- 

 gesting that exogastrulation is not primarily concerned with a specific 

 vegetal gradient. 



There is, however, no evidence of such reversal in cells of entodermized 

 ectoderm. They retain their original relations to other parts and to the 

 blastocoel and show no tendency to invaginate, even in the most com- 

 plete recoveries. Conceivably, either their failure to invaginate may be 

 due to insufficient activation and susceptibility to the surface-interior 

 differential, or in inhibited forms, as all these are, the differential between 

 blastocoel and exterior may be insufficient to bring about reaction or in 

 some cases may be altered in character by the dissociated cells in it. But 

 whatever the physiology of exogastrulation, it is sufficiently evident that 

 neither true exogastrulation, that is, evagination of prospective entoderm, 

 nor pseudoexogastrulation, development of an external entoderm from 

 entodermized ectoderm, are specific effects of lithium. 



RECONSTITUTION IN EXOGASTRULATION 



Development of exogastrulae with more or less entodermization of pro- 

 spective ectoderm may involve extensive reconstitution. All ectodermal 



