EMBRYONIC RECONSTITUTIONS 521 



not complete agreement as regards presence of a Bildungszentrum in all 

 insects investigated. According to the gradient concept, it is to be expect- 

 ed that ligature and cautery will be more effective in some species than in 

 others in obhterating the polar gradient at more anterior levels. This is 

 the case with posterior section in different planarian species. 



As regards the prothoracic or maxillar-prothoracic differentiation cen- 

 ter and its significance, or even its presence in certain forms, there seems 

 also to be difference of opinion. If heads or anterior head regions can de- 

 velop in its absence, it does not appear to be essential unless it is present in 

 the cortical cytoplasm before nuclei reach the cortex, as some believe it to 

 be. The question whether it possesses inductive capacity or is merely a 

 region in which cell formation is more rapid and susceptibility greater than 

 elsewhere seems still to be open. Certainly prothoracic regions can be re- 

 constituted in other regions of the egg than the original presumptive pro- 

 thoracic region. If there is a prothoracic region of primary dominance, the 

 head is apparently secondary in origin; that is a unique polar pattern. 

 However, axiate pattern may be present in the cortex before nuclei reach 

 it; the head region may be a primary feature of that pattern; and the pro- 

 thoracic differentiation center a secondary development associated with 

 nucleation of the cortex. Early insect development remains an extremely 

 interesting problem. 



RECONSTITUTION IN EARLY FISH DEVELOPMENT 



Development of isolated blastomeres of the holoblastic lamprey egg in- 

 dicates presence of a dorsal inductor region more or less similar to that of 

 amphibians.'' Early embryonic stages of the meroblastic fish eggs show 

 high capacity for reconstitution. The cleaving blastoderm of Raja, sepa- 

 rated by section into fourths, may give rise to several embryonic primor- 

 dia, even though the separated parts gradually unite again to a single disk 

 (Eismond, 1910). However, a differential appears to be present in the 

 blastoderm, for the primordia develop from median and lateral regions of 

 one half, none developing from the other half ; and retarded development 

 and early degeneration of the lateral primordia after reunion of parts sug- 

 gests that their further development is inhibited by dominance of more 

 nearly median primordia, which probably represent higher gradient levels 

 of the blastoderm. 



In teleosts normal embryos may develop from parts of the germinal 

 disk, from 1/2 blastomeres, from two 1/4 blastomeres, from six 1/8, and 



^Bataillon, 1900a, b; Montalenti e Maccagno, 1935. 



