692 PATTERNS AND PROBLEMS OF DEVELOPMENT 



for invertebrate asymmetries. A few points that may have some bearing 

 on the problem are briefly noted. 



In both urodele and frog developmental stages with open neural folds 

 removal and reimplantation in the same place, with anteroposterior re- 

 versal, of a dorsal rectangular piece of sufficient size from the middorsal 

 region, including presumptive neural tissue and underlying inductor ma- 

 terial, result in reversal of asymmetry; if the piece is merely removed, 

 asymmetry is not reversed. ^^ Other experiments show that altered orien- 

 tation of prospective neural tissue does not influence asymmetry; conse- 

 quently, it seems evident that the archenteric roof is a predominant factor 

 at the stage concerned in determining the laterality of asymmetry, but it 

 is not the only factor. These experiments throw no light on the origin of 

 asymmetry. 



Reversal of polarity in the undivided frog egg by reversal of position 

 in relation to gravity does not reverse asymmetry (Hammerling, 1927), 

 nor does separation of right and left 1/2 blastomeres in a urodele by 

 gradual constriction with a hair ligature (Mangold, 1 92 1) . Gradual partial 

 constriction by ligature of the urodele blastula in the median plane gives 

 a somewhat different result. Monsters with more or less anterior duplica- 

 tion develop, the left member always with normal asymmetry, the right 

 usually with situs inversus. ^^ If constriction is continued to complete 

 separation of right and left halves, the left half develops with normal 

 asymmetry, the right about equally with normal and reversed asym- 

 metry (Ruud und Spemann, 1922). Division of the heart primordium into 

 right and left halves at the tail-bud stage results in development of two 

 hearts, the left with normal, the right with reversed, asymmetry (Ekman, 

 1925). These data bring out two important points: first, asymmetry ap- 

 parently develops gradually, for reversal of polarity in the undivided egg 

 and separation of right and left halves at the two-cell stage do not alter 

 asymmetry, but with separation at the blastula stage the right half is 

 often reversed; second, the laterality of asymmetry is apparently more 

 stable in the left than in the right half, as if there were on the left side a 

 region dominant as regards asymmetry. A gradually developing activity 

 gradient with high end on the left side is postulated by Huxley and 

 De Beer (1934, pp. 73-82). 



3^ Spemann, 1906, 1918; Pressler, 191 1; R. Meyer, 1913. 



37 Spemann und Falkenberg, 1919. Anterior duplications in fishes, if they do not extend 

 posteriorly beyond a certain level, often show situs inversus in the right member, as noted by 

 Stockard, 192 1, and by other authors. 



