CELLULAR DIFFERENTIATION" AND INTERNAL ENVIRONMENT 



85 



inhibition of the spread of the apical tuft 

 by the presence of micromeres at the op- 

 posite pole. In contrast with the mosaic 

 eggs referred to above, the sea-urchin egg 

 gives much more evidences of interaction 

 between parts after development has 

 started, but there is no such complete 

 isotropy in the sea urchin egg as Driescli 

 supposed. The material of the vegetative 

 portion of the egg is different from that of 

 the animal portion, and some of each is 

 necessary for normal development. 



According to the Runnstrom-Horstadius 

 theory, there are in the egg two material 

 gradients oriented in opposite directions; 

 one diminishes as the other increases from 

 one pole to the other. At the animal pole, 

 the "most animal" region of the egg, the 

 power to form structures characteristic of 

 that region is most intense; at the vegeta- 

 tive pole, the ' ' most vegetative ' ' region, the 

 power to invaginate and form structures 

 normally arising there (archenteron, mes- 

 enchyme) is at a maximum. When any 

 portion of the egg is cut out there is a 

 tendency toward a rearrangement of these 

 materials so that the gradients are restored 

 approximately to their original scale. Be- 

 fore the third (equatorial) cleavage mem- 

 brane is established this might take place 

 by rearrangement of coarsely particulate 

 material, but the fact that the phenomenon 

 occurs even after the egg is divided into 

 many cells indicates that the reestablish- 

 ment of the gradients after disturbance is 

 brought about by subtler changes in the egg 

 materials. 



While there are many processes in the 

 development of the sea urchin that are 

 either of a self -regulatory character or are 

 dependent upon the internal environment 

 for realization, the marked regional differ- 

 ences in the several layers of the egg as 

 regards their powers of differentiation show 

 that the mosaic quality of this egg is not 

 negligible. 



The amphibian egg, thought by Roux 

 (1888) to be the archetype of mosaic de- 

 velopment, is now known to be about as 

 regulable as the sea-urchin egg. However, 

 all amphibian eggs that have been studied 



show a certain amount of germinal locali- 

 zation at the time cleavage begins — a corti- 

 cal layer, the gray crescent, and peculiar- 

 ities of yolk and pigmentation in the two 

 hemispheres. These eggs are enclosed in a 

 tight membrane and are very fluid, so that 

 removal of parts of the unsegmented egg 

 by pricking to test their function in de- 

 velopment has not yielded results that can 

 be clearly interpreted (Brachet 1923). On 

 the other hand, the effects of separation 

 and rearrangement of cell constituents by 

 means of the centrifuge and by gravitation 

 (inversion of eggs) have given results of 

 interest. 



When the egg of a frog is turned there 

 is a marked rearrangement of constituents. 

 The cortical layer, which holds most of the 

 pigment, and the main part of the gray 

 crescent remain in position, and the main 

 mass of heavy white yolk rotates as a unit. 

 The position of the dorsal lip of the blasto- 

 pore in such eggs indicates that it is formed 

 in definite relation to the gray crescent 

 (cortical field) and the yolk mass (Weig- 

 mann 1927; Dalcq and Pasteels 1937). In 

 other words, turning the egg does not result 

 in indiscriminate mixing of the elements 

 but in an orderly rearrangement, which 

 may be such that a normal embryo may de- 

 velop from it. When the egg is inverted 

 in the two-cell stage the rearrangement 

 takes place independently in the two cells. 

 This may result in the formation of two 

 separate dorsal blastoporic lips followed by 

 the development of a twin embryo (Schultze 

 1894). All of this goes to show that even 

 in the earliest stages, development is af- 

 fected by the interaction of cell constituents 

 according to an orderly topographic ar- 

 rangement. The internal environment of 

 the cell constituents within the cell thus 

 plays an important role. The nature of 

 the reactions is, however, in total obscurity. 



The amphibian egg shows perhaps better 

 than any other the combination of mosaic 

 and regulative qualities. The normal 

 scheme of germinal localization has been 

 mapped in great detail by vital staining, 

 first applied in a wholly satisfactory and 

 exhaustive way by Vogt (1925, 1929). The 



