264 EARLY ECHINODERM DEVELOPMENT 



The purpose of this brief review is to attempt an examination 

 of cmTent information concerning patterns and gradients of cyto- 

 plasmic particles in echinodemi embryos and to assess some of 

 the recent hypotheses concerning their fmictions in early differ- 

 entiation. 



Cytoplasmic Particles and Initial Stages of Development 



As Tyler remarks ( 1955 ) : "An understanding of the factors 

 that endow egg and sperm with the ability to unite and produce 

 a new individual may be expected to depend largely on knowl- 

 edge of the manner of formation of the gametes." An extensive 

 literature exists describing the results of various cyto- and histo- 

 chemical tests performed on cells in various stages of gametogen- 

 esis (for a review of earlier literature, see Brachet, 1950). Al- 

 though there is information on the origin of ribonucleoproteins 

 in the gonial cells of several invertebrate species (Arvy, 1950; 

 Nigon and Delavault, 1952; Faure-Fremiet et ah, 1950; Fautrez- 

 Firlefyn, 1951; Panijel, 1951) and a great deal of discussion about 

 the possible participation of these substances in yolk formation 

 (cf. Wittek, 1952; Urbani, 1955), there seems to be no evidence 

 that ribonucleic acid, demonstrable by the usual cytochemical 

 tests, is associated for the most part with sedimentable fractions 

 of oocytes or unfertilized eggs. Brachet and Chantrenne (1942) 

 have reported about 20% to 40% of the ribonucleic acid to be 

 associated with granules in amphibian egg homogenates, but the 

 granule-free fragments of sea urchin eggs obtained by centrifu- 

 gation were reported to be rich in ribonucleic acid (Harvey and 

 Lavin, 1944). 



It has long been known, however, that gonial cells and mature 

 gametes contain cytoplasmic elements identical with those found 

 in adult cells (Wilson, 1928). Recent observations with phase 

 contrast and electron microscopy confirm the existence of "typi- 

 cal" mitochondria and/or Golgi bodies in the protoplasm of un- 

 fertilized sea urchin eggs (Shaver, 1955), of germ cells of Helix 

 aspersa (Beams and Tahmisian, 1953), and of frog eggs (Nath 

 and Malhotra, 1954 ) . The flow of these particulate elements from 

 nurse cells into the maturing egg has been described (Wilson, 



