268 EARLY ECHINODERM DEVELOPMENT 



lization to see if any indications of de novo formation of mito- 

 chondria could be seen in later stages. 



2. The second point concerning the granule-free nature of the 

 clear fragments of centrifuged sea urchin eggs concerns the ob- 

 servation by Lansing and Hillier ( 1954 ) that, in electron micro- 

 graphs of stratified Arbacia eggs, two distinct layers of mito- 

 chondria could be seen, one of higher density than the other. It 

 was pointed out ( Palade, 1954 ) that the light mitochondria might 

 have been trapped just below the lipid layer. The observations of 

 Shaver concerning the close proximity to the oil cap of the small 

 fibrils and the light mitochondria seem to agree with this sugges- 

 tion. 



Admittedly, it is very difficult to be certain that the exact con- 

 ditions of Harvey's experiments are being duplicated in attempts 

 to repeat them, but the report by Harvey of granular elements 

 in the clear quarters, together with the observations noted above, 

 suggest that a reinvestigation of the development of clear frag- 

 ments might be profitable. 



But aside from the question of the presence or absence of cyto- 

 plasmic particles in egg fragments which produce normal em- 

 bryos (which is of obvious physiological interest), it is evident 

 that there are no patterns of distribution of mitochondria and/or 

 microsomes in the unfertilized sea urchin egg which have deter- 

 minative significance. It is equally evident, from the results of 

 micrurgical operations made in various planes of eggs and early 

 embryos of both mosaic and regulatory eggs, that determinative 

 patterns are established at various times before fertilization, or 

 shortly thereafter (see Watterson, 1955, for a recent review of 

 these experiments ) . The ground substance or the cortex, or both, 

 have been suggested as the site of such a localization, the latter 

 having attracted especial interest because of the presence in it of 

 granular structures, in sea urchin eggs, which undergo striking 

 changes at fertilization (Hars^ey, 1910; Moser, 1939). 



In the experiments by Harvey ( 1946 ) , it was observed that 

 cortical granules in Arbacia were not displaced by the gravita- 

 tional forces used. Motomura (1949), using much higher forces, 

 reported that the axis of differentiation in eggs of two sea urchin 



