528 PATTERNS AND PROBLEMS OF DEVELOPMENT 



cally unaltered. In later stages, when gradient pattern has developed, fur- 

 ther removal of a part is followed by activation and increased growth 

 near the cut surface, and regeneration results. 



Some parts of the amphibian and other embryos, isolated after a cer- 

 tain stage under proper conditions, are capable of independent or self- 

 differentiation, that is, they are able to continue for a time their original 

 course of development. It is commonly assumed that such parts also dif- 

 ferentiate independently when parts of the intact organism and that, in so 

 far as parts attain this condition, the organism becomes a ''mosaic" of in- 

 dependent parts. This conclusion does not seem entirely justified. Even 

 though differentiation is independent after isolation, it may not have been 

 before. In other words, self-differentiation of an isolated part may repre- 

 sent more than it accomplishes independently without isolation. The fact 

 that it can differentiate independently is not proof that it does so in the in- 

 tact organism. 



RECONSTITUTION AND PATTERN IN THE AVIAN EMBRYO 



Morphological and material aspects of developmental pattern in the 

 chick have been subjects of investigation by various methods and of con- 

 siderable controversy for many years. Local injury — mechanical, elec- 

 trolytic, or by radiation — has been extensively used in attempts to throw 

 light on the problem, and modifications resulting from subjection of em- 

 bryonic stages to various chemical and physical agents have been de- 

 scribed and analyzed. In recent years data on differential susceptibiHty 

 and differential dye reduction, transplantations of parts to other embryon- 

 ic regions or to the chorio-allantois, as a supposedly neutral site outside the 

 embryo proper, and explantations to plasma clots with embryonic extract 

 have given information concerning some of the physiological features of 

 pattern and concerning regional and chronological differences in develop- 

 mental capacity or potency under certain experimental conditions and 

 their relations to the general pattern. Some of the evidence from these ex- 

 periments is briefly reviewed. 



Cell migrations are very slight during the first lo hours of incubation, 

 and extensive reconstitution is possible at these stages. Lesions produced 

 electrolytically or by ultra-violet radiation in the median posterior region 

 of the blastoderm result in various anomalies, among which are more or 

 less complete embryonic duplications and supplementary heads (Twiessel- 

 mann, 1938). The duplications are regarded as determined by the effect 

 of experimental procedure on the inductor — inhibition or killing of the 



