428 PATTERNS AND PROBLEMS OF DEVELOPMENT 



fragments can be fertilized and show more or less development, the color- 

 less halves becoming plutei, the pigmented halves containing only the 

 male nucleus, sometimes forming blastulae and occasionally plutei. Un- 

 fertilized centrifuged halves without nucleus, vv^hen artificially activated, 

 may cleave and form blastulae. Centripetal halves, artificially activated, 

 usually develop normally. When centrifuged after fertilization, centripetal 

 halves, containing both nuclei, develop, but plutei are not normal; cen- 

 trifugal halves do not develop. ^^ Evidently there is extensive reconstitu- 

 tion in those fragments which develop, but exactly how the pattern is 

 altered is not known. These, like many other centrifuge experiments, sug- 

 gest that the pattern in these eggs is chiefly or primarily cortical but 

 that protoplasmic content inside the cortex, as distinguished from granu- 

 lar inclusions, is perhaps of significance in maintaining sufficient cortical 

 activity for development. 



In most animal eggs a considerable degree of stratification by centri- 

 fuging may occur without essential alteration of developmental pattern, 

 though, as noted above, it may alter position of polar-body formation by 

 displacing the nucleus or maturation spindle. However, eggs of certain 

 ascidians stratified by centrifuging show dislocation of pattern of tissues 

 and organs (Conklin, 1931). 



Perhaps the most interesting, certainly the most extensively investi- 

 gated and discussed, case of alteration of animal developmental pattern 

 by gravity and centrifugal force is that of the amphibian egg and early 

 embryo. Only brief consideration of some of the more important points 

 is attempted here. In amphibian eggs and early cleavage stages, main- 

 tained in inverted or partly inverted positions with respect to gravity, or 

 centrifuged in these positions, more or less complete reversal in position 

 of heavier and lighter substances occurs. The heavier yolk accumulates 

 in the original apical region, displacing the lighter parts of the cytoplasm 

 to the upper, originally basal region, and certain alterations of pattern 

 result. ^^ 



Development of double monsters from the inverted two-cell stage, first 

 observed by Schultze, was usually interpreted as the result of decrease 



35 Harvey, 1932, 1936, 1939. 



36 Born, 1885; Schultze, 1894; Wetzel, 1895, 1896; Chiarugi, 1898; Tonkofi, 1900, 1904; 

 Bagini, 1923; Schleip und Penners, 1925, 1926; Wiegmann, 1926, 1927; Hammerling, 1927; 

 Penners und Schleip, 1928; Penners, 1929; Wittmann, 1929; Dalcq et Pasteels, 1938; Pasteels, 

 1938; also, omitted from bibliography, Pasteels, 1938, "Recherches sur les facteurs initiaux 

 de la morphogenese chez les Amphibiens Anoures, I," Arch. Biol, 49. For a general discus- 

 sion with bibliography see Schleip, 1929, pp. 584-90, 696-715. 



