EMBRYONIC RECONSTITUTIONS 541 



erated; but whether, or to what extent, it may be altered, is not known. 

 Fusion among the several eggs in a capsule, either before or during cleav- 

 age, has been observed in another nemertean. Linens ruber; and fusion of 

 two eggs may result in normal gastrulae with cells approximately double 

 size (Nusbaum und Ochsner, 1913). 



The giant eggs of Ascaris megalocephala undoubtedly result from fusion 

 of two or more eggs. Completeness of fusion varies widely ; the giant eggs 

 may be doubly fertilized, form two sets of polar bodies at different points, 

 and show more or less double cleavage; or in some cases they may de- 

 velop as a single individual. '^ Apparently the variations depend on the 

 relations of the apicobasal axes, on the degree of physical union, and per- 

 haps on other factors. Supposedly, development as a single individual 

 occurs only when polarities of fusing eggs are parallel and identical in 

 direction. 



Fused unfertilized and fertilized sea-urchin eggs and early developmen- 

 tal stages give forms ranging through various degrees of twinning to single 

 individuals. Skeletal duplications or excessive skeletal development and 

 duplications of the archenteron are frequent.^'' Fusions of more than two 

 individuals are usually highly abnormal and die early. Single giant larvae 

 may develop from fusions with axes known not to be parallel, and twin- 

 ning may occur when axes are parallel. Extensive shiftings and rotations 

 of the components, either toward or away from paralleHsm, may take 

 place. The larger component may dominate the smaller to such a degree 

 that suppression of its development, reduction in size with translocation 

 of cells, and even complete incorporation into the body of the dominant 

 member may result. Translocation of mesenchyme cells to the dominant 

 member indicates that the regions of its ectoderm which determine locah- 

 zation of mesenchyme are more effective than those of the subordinate 

 member. At present it appears difficult to interpret the observed results 

 except in terms of dynamic factors. Investigation by means of differential 

 dye reduction of the gradient relations and the changes which they under- 

 go in these fusions, particularly in cases of suppression and absorption of 

 one member, would undoubtedly be of interest. 



First cleavage stages of Ascidiella aspersa united in various orientations 

 develop as more or less completely double forms, often with organ anoma- 

 hes and dislocations, or as forms apparently single externally but more 



^s Sala, 1895; Zur Strassen, 1898, 1906; Kautsch, 1913. 



*' Driesch, 1893, 1900, 1903, 1910; Morgan, 1895c; Garbowski, 1904; Bierens de Haan, 

 1913d, b; Goldfarb, 19146, 1915, 1917; von Ubisch, 1925(2; Schleip, 1929, pp. 484-95. 



