J. R. SHAVER 271 



been invoked to explain the appearance of specific morphological 

 properties in cells. As Wilson (1928, p. 1085) states: "More ac- 

 cessible to investigation ( than theories of invisible pangens' ) are 

 hypotheses which assume differentiation to be effected by the 

 transformation of visible cytoplasmic granules or other definite 

 bodies, arising either by migration from the nucleus, or independ- 

 ently in the cytosome. . . ." 



Equally influential has been the concept of differentiation 

 which expresses regional determination of the embryo in terms 

 of gradients of types or intensity of metabolism, or of specific 

 substances. The manifestations of this idea, as applied to various 

 embryonic or adult differentiating tissues, are protean. The re- 

 views of Lindahl (1942), Brachet (1950), Horstadius (1949), 

 and Child ( 1941 ) present the evidence for and criticisms of gra- 

 dient concepts as applied to the differentiation of echinoderms. 



More recently, a theory of differentiation for the sea urchin 

 has been advanced by Gustafson (1954), which embodies fea- 

 tures of both the above mentioned concepts. It incorporates 

 within the double-gradient theory of Runnstrom (1928a) and 

 Horstadius ( 1949 ) , the idea that cytoplasmic particles, specif- 

 ically the mitochondria, act as primary agents in establishing the 

 basic embryonic shape. Gustafson assumes that mitochondria in 

 actively differentiating regions produce fibrous structure proteins, 

 which are responsible for such transformations as cell stretching 

 and the appearance of the apical tuft. 



The principal observations and experiments on which Gustaf- 

 son's concept is based are: 



1. Mitochondria in the egg and early stages are represented 

 mainly as "precursors," described as "small granules," which are 

 sensitive to the action of lithium during a certain period. 



2. The number of mitochondria per se in early stages is low. 

 Gustafson and Lenicque ( 1955 ) have described a small increase 

 in mitochondria during cleavage and blastula stages, which is 

 followed by a rapid and large increase in the number of these 

 particles at the mesenchyme blastula stage. 



3. Counts of the mitochondria in mesenchyme blastulae and 

 young gastrulae, along the animal-vegetal axis, indicate, accord- 



