EMBRYONIC RECONSTITUTIONS 505 



restricted potency. Further experiment with developmental stages of this 

 group is necessary for definite conclusions. 



ECHINODERMS 



Some of the earliest experiments on embryonic reconstitution were per- 

 formed with sea-urchin eggs and cleavage stages, and their high reconsti- 

 tutional capacities aroused great interest and led to extensive further ex- 

 perimentation. These early experiments appeared particularly significant 

 because they came at a time when the predeterministic Weismannian 

 theory of development was current and when studies of cell lineage in an- 

 nelids and mollusks indicated a high degree of predetermination in those 

 forms. ^ 



Early stages of sea-urchin development were regarded as constituting 

 a harmonious-equipotential system by Driesch (1899, 1901), but with the 

 progress of experiment some data seemed to point to a different conclu- 

 sion. In early experiments parts of sea-urchin eggs and blastomeres were 

 isolated by section or by shaking, and it was shown by various experi- 

 menters (Boveri, Driesch, Loeb, Morgan, Delage, and others) that nu- 

 cleated and even nonnucleated fragments of unfertilized eggs above a cer- 

 tain size could be fertilized with sperm of the same or of another species 

 and might develop and that whole eggs and nucleated fragments could 

 be activated to parthenogenetic development by various agents. These 

 experiments, however, gave little or no information concerning develop- 

 mental pattern because the portion of the egg represented by a particular 

 fragment was usually not known. They did indicate, however, that con- 

 siderable reconstitution was possible. In recent years development of 

 techniques of isolation of blastomeres and of local vital staining has made 

 it possible to obtain much more exact information concerning regional 

 differences in reconstitution in these eggs and embryos than earlier meth- 

 ods permitted. Single blastomeres and groups can be isolated as desired, 

 locally stained for orientation or identification, and transplanted in de- 

 sired locations and combinations.-' 



^ In view of the more definite results obtained in recent years with improved techniques, 

 discussion of the earher work appears unnecessary. Among the numerous workers who con- 

 tributed to this field, the following are mentioned with dates indicating approximately the 

 periods covered by their papers, but the papers are given in the Bibliography only when par- 

 ticularly referred to in the text. Driesch, 1891-1923; Boveri, 1889-1914; Morgan, 1894-1908; 



Ziegler, 1896-1925; Garbowski, 1904-10; Jenkinson, 19116; von Ubisch, 1913 ; Runnstrom, 



1914 . For bibliography to 1928 see Schleip, 1929. 



i See, e.g., von Ubisch, 1925a, b, 1929, 1932a, b, 1936; Horstadius, 1928a, b, 1935, 1936a, b, 

 1937a; Horstadius und Wolsky, 1936; also pp. 436-46. 



