444 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



reversal of polarity (Fig. 148, D-F). As far as the experimental evidence 

 goes, the invaginations induced by implanted micromeres may conceiv- 

 ably be due to an activation above the ectodermal level, such as appar- 

 ently occurs normally at the time of immigration of primary mesenchyme, 

 or to a specific effect. Incidentally, it may be noted that apical invagina- 

 tions, in appearance much like those figured by Horstadius, are present 

 not infrequently in secondary modifications of differentially inhibited 

 forms, even in exogastrulae. Although they appear at the extreme apical 



Fig. 148, A-F. — Implantation of four micromeres into apical pole. A-C, implantation in 

 whole embryo with development of small entodermal invagination, persisting in apical region 

 of pluteus; D-F, implantation of micromeres into apical region of apical half-blastula with 

 complete reversal of polarity; stained micromeres and apical region of half indicated by shad- 

 ing in D (from Horstadius, 1935). 



pole, they have been regarded as stomodeal invaginations. To account 

 for reconstitution of an apical region from the original basal region of 

 the apical half-blastula (Fig. 148, D-F), either in terms of activity gra- 

 dients or concentration gradients, requires special assumptions or hypoth- 

 eses. It may conceivably represent a physiological isolation, resulting 

 from alteration in the original apical region. When formed, the micro- 

 meres apparently represent the low end of the primary gradient; their 

 implantation apically may interfere considerably with the primary gra- 

 dient in early stages before their supposed activation and so may permit 

 physiological isolation of the basal region and reconstitution of an apical 

 region there. In terms of concentration gradients interpretation seems at 



