RECONSTITUTIONAL PATTERNS IN EXPERIMENT 429 



or elimination by the cytoplasmic movements of relations between the 

 two blastomeres, so that reconstitution resulted in a more or less com- 

 plete, instead of a half -embryo, from each. Later experiments showed that 

 this interpretation was not adequate, for it was found that double em- 

 bryos might develop from inverted undivided eggs, from four-cell, and 

 even from eight-cell stages, and occasionally triple monsters developed 

 from two-cell stages (Wetzel, 1896; Schleip und Penners, 1925). Most 

 authors are agreed that the region of the gray crescent, which normally 

 becomes the dorsal lip of the blastopore, the ''organizer" region, does not 

 change its position in inverted eggs; and, according to Penners and Schleip 

 (1928), sinking of the white yolk occurs in different ways in different 

 eggs, and portions of it may remain in the original basal region or at the 

 surface along the cleavage furrows present. These authors maintain that 

 gastrulation and organization are not necessarily dependent on the origi- 

 nal dorsal lip region but may result from localization of such regions where 

 a band or mass of white yolk is in contact with other cytoplasm. Double 

 embryos arise by localization of dorsal lip regions and invagination in op- 

 position directions on both sides of a band or streak of yolk, left behind 

 as the yolk sank. Gastrulation and organization may also be localized 

 in relation to other yolk masses that failed to sink or reached the surface 

 at the lower, originally apical, pole. Evidently the region of the gray 

 crescent is not definitively determined as the only dorsal lip region and 

 organizer up to four-cell and eight-cell stages. Other regions are capable 

 of becoming organizers and of determining gastrulation and position and 

 axial direction of the neural plate. Locahzation of gastrulation and neural 

 plate are related to gravity or centrifugal force only in so far as these 

 agents alter local relations of yolk masses and cytoplasm. 



Further analysis by means of partial inversion confirms the work of 

 Penners and Schleip as regards the significance of contact of cytoplasm 

 and yolk masses for localization of invagination and of the neural in- 

 ductor or organizer and leads to the further conclusion that localization 

 of the blastopore or blastopores is as near the original prospective dorsal 

 lip as the relations of cytoplasm and yolk permit (Pasteels, 1938; Dalcq 

 et Pasteels, 1938). In frog eggs maintained in inverted position by com- 

 pression between slides a blastopore lip forms at the edge of peripheral 

 yolk but, because of the compression, does not invaginate, fades out in a 

 few hours, and another or others may be formed (Pasteels, 1939). Dalcq 

 and Pasteels conclude that the essential pattern of amphibian develop- 

 ment is a primary apicobasal cytoplasm-yolk gradient and a dorsal cor- 



