SOME OBSERVATIONS ON CYTOPLASMIC 

 PARTICLES IN EARLY ECHINODERM 

 DEVELOPMENT 



JOHN R. SHAVER:* kerckhoff laboratories of biology, 



CALIFORNIA INSTITUTE OF TECHNOLOGY, 

 PASADENA, CALIFORNIA 



An embryonic system could be defined within the framework 

 of our present concepts of cellular metabolism as one which, in 

 addition to the demands of growth and maintenance, has to meet 

 the unique exigencies of differentiation (cf. recent discussions of 

 Steinbach and Moog, 1955, and Boell, 1955 ) . Our understanding 

 of the ways in which differentiated cells of metazoan organisms 

 carry on such vital functions as energy transfer and protein syn- 

 thesis has been enormously extended in recent years by studies 

 on the structure and function of various kinds of cytoplasmic 

 formed elements. It does not seem surprising, therefore, that 

 numerous suggestions have come from embryologists concerning 

 the role of these subcellular entities in embryonic differentiation 

 (for recent reviews see Brachet, 1950, 1952; Gustafson, 1954). 



But the information about the interrelationships of cytoplasmic 

 particles and other cell constituents in the life of the cell is still 

 largely descriptive, especially as they concern the intact cell (cf. 

 Porter, 1955 ) . To assign a dominant, if not a unique, place to one 

 or another of the granule populations of protoplasm in the series 

 of transformations leading to specific cell types necessitates an 

 identification of visible structures with causality which still seems 

 very far from being able to be made. 



* Special Research Fellow, National Cancer Institute, United States Pub- 

 lic Health Service. Present address: Department of Zoology, Michigan State 

 University, East Lansing, Michigan. 



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