542 PATTERNS AND PROBLEMS OF DEVELOPMENT 



or less completely duplicated internally ; or in a single case among more 

 than a hundred developing, a single individual resulted from union with 

 apical pole of one component in contact with basal pole of the other (von 

 Ubisch, 19386). The author holds that the general organ-forming regions 

 develop according to their prospective significance but that within them 

 there may be extensive reconstitution. Two such regions may give rise to 

 a single organ system, or one may form two systems. 



Homoplastic and heteroplastic fusions of two two-cell stages have been 

 accomplished with interesting results in species of the urodele Triton. 

 After removal of membranes the blastomeres of the two-cell stage become 

 almost spherical and are connected only by small areas. At this stage one 

 pair of naked cells is laid crosswise on the other, and the four cells come 

 gradually to lie in a plane, the blastomeres of one component alternating 

 with those of the other. From these fusions one to four axial systems may 

 develop. ^^ A single embryo resulting from a heteroplastic fusion is a chi- 

 mera, that is, composed of cells of two species. The results of these ex- 

 periments are interpreted in terms of the relation of the first cleavage 

 plane to the median plane and the consequent positions in the fused pair 

 of the dorsal inductor tissue, assuming that this inductor region is already 

 more or less definitely locahzed and determined at the two-cell stage. 

 Since the first cleavage is known to make any angle with the median 

 plane, the dorsal inductor region of each two-cell stage may be divided by it 

 into equal or unequal parts or be entirely in one cell, and the relation of 

 cleavage plane and median plane may be different in the two fused com- 

 ponents. Consequently, neural induction and embryonic axes will appear 

 in the fused pair in various positions and combinations of parts. The re- 

 sults agree with expectation and are in accord with other experiments 

 (chap, xii) in indicating that a part of the inductor may reconstitute to a 

 whole, that induction is not species-specific, and that such symmetry 

 pattern as may be present at the two-cell stage may undergo extensive 

 alteration in the reconstitutions resulting from the fusions. 



CONCLUSION 



It appears that embryonic reconstitutions in many animals do not 

 differ very greatly from reconstitutions in adults of the lower inverte- 

 brates, except that they are usually more narrowly Hmited. They com- 

 monly show definite relations to the original pattern, but sometimes that 

 is completely obliterated and new pattern determined. The embryonic 



*7 Mangold, 1920; Mangold und Seidel, 1927; also Spemann, 1938, pp. 271-77. 



