THE AXIAL GRADIENTS IN HYDROZOA. or 



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whole oocyte, though the absence in most cases of any visible 

 basis for orientation of the fragments makes complete demon- 

 stration of identity impossible. In the oocyte of Stomotoca the 

 gradient shows the same relation to free and attached poles of 

 the cell as in Phialidium. 



It has been repeatedly pointed out that the various lines of evi- 

 dence concerning the gradients indicate very clearly that they 

 are associated with quantitative differences in physiological condi- 

 tion involving metabolism as an important factor. 1 In the 

 li-ht <>f this evidence the inference is justified that the differ- 

 entials in susceptibility, reduction and differential staining in 



e liy<! oocytes are indicators of a quantitative physi- 



'I'ocal gradient in the cytoplasm. The relation of this gradient 

 to the free and attached ends of the oocyte in the gonad suggests 

 that it is determined by the differential exposure of the cell. The 

 intake of i n and elimination of Co 2 undoubtedly occur more 



lily at the free end than elsewhere, and there is every reason 

 that the persistence of such a differential during the 



A tli peril 'd will establish a physiological gradient, primarily 

 quantitative in character, involving metabolic and physical factors. 

 h: ^h"tt. the facts at hand indicate, though they do not demon- 



te that tlu- physiological polarity of the oocyte is determined 

 by its differential exposure in the gonad. 



The persists after the egg is shed, the polar bodies are 



high end, i.e., the pole of highest susceptibility and 

 redurin- p. i\ver, and the first cleavage furrow cuts through the 

 fn >m this [Mile (Fig. 12). During cleavage and early blastula 

 adient is continuously present, undoubtedly the same as 

 that <>f the early stages, but absence of definite landmarks makes 

 complete demonstration of identity impossible. As soon as di- 

 rected Ino mint inn and elongation of the blastula begin, we find 

 that the gradient is apico-basal, the high end being at the apical 

 pole of the blastula (Fig. 13) and the planula (Tig. 14). The 

 facts leave little mom for doubt that the gradient, apparently de- 

 nined by ditYerential exposure in the oocyte, represents the 

 polar axis of the egg and larva. Moreover, maturation, cleavage, 



i F,>r discussions of this evidence see Child, '20, '21, Chap. II.; '235 ; '--4. 

 Chap. VII. ami literature cited in these publications. 



