524 PATTERNS AND PROBLEMS OF DEVELOPMENT 



embryo may later become more or less a whole, either by reconstitution or, 

 in case the other blastomere was not completely killed, by spread of nuclei 

 into it and by cell formation from its cytoplasm (postgeneration). This 

 apparent reconstitution of a half to a whole was the subject of much con- 

 troversy. Combining killing of one blastomere with maintenance in in- 

 verted position, it was found that the living blastomere might develop as a 

 whole, supposedly in consequence of a more complete isolation of the liv- 

 ing from the killed blastomere through the rearrangement of materials by 

 gravity (Morgan, 1895^). The blastomeres are too fragile to permit direct 

 separation, but they may be separated by gradual ligature, or one may be 

 removed by suction. With these procedures the 1/2 blastomere may de- 

 velop as a whole, or with incomplete separation by ligature various de- 

 grees of twinning may result.'^ 



Partial separation of undivided eggs by ligature shortly after fertiliza- 

 tion leaves one of the two parts without a nucleus. The nucleated portion 

 develops, the nonnucleated does not; but if the ligature does not complete- 

 ly separate the two parts, a cleavage nucleus may sooner or later pass 

 through the constriction into the nonnucleated part, and this may then 

 begin to develop. Such cases may result in partial or complete twinning, 

 the originally nucleated portion being in advance of the other. Moreover, 

 it was found that complete separation of the first two blastomeres by liga- 

 ture or otherwise gave different results according to relation of plane of 

 first cleavage to the median plane. When the first cleavage plane coincides 

 with the median plane, both right and left half are able to develop as 

 wholes; but when dorsal and ventral halves are separated, the dorsal de- 

 velops as a whole, but the ventral forms only a rounded mass, showing 

 some development of germ layers but without axial organs. The dorsal in- 

 ductor region is responsible for this difTerence in development. With me- 

 dian first cleavage both blastomeres contain half of this region; and, since 

 the isolated half is able to induce whole development, each may develop 

 as whole. When the first cleavage is frontal, the inductor is wholly in the 

 dorsal blastomere. Apparently the same differences result from ligature 

 of the undivided egg in different planes.'-^ These differences in develop- 

 ment are regarded as indicating early locahzation of the dorsal inductor 



"Endres, 1895; Herlitzka, 1896; Spemann, igoib, 1902, 19036; McClendon, 1910; Rand, 

 1925; G. A. Schmidt, 1930, 1933. 



13 Spemann, igoib, 19036, 1914, 1928, 1936, pp. 16-18; Fankhauser, 1930a, b; also Streett, 

 1940, "Experiments on the organization of the unsegmented egg of Tritiiriis Pylirrogaster," 

 Jour. Exp. Zool., 85. 



