280 EARLY ECHINODERM DEVELOPMENT 



These investigations are reviewed in Wilson ( 1928 ) ; more recent 

 hypotheses are summarized in Lindberg and Ernster's recent re- 

 view (1954). 



The earher results included most of the logical possibilities that 

 can be imagined for the duplication and segregation of cytoplas- 

 mic particles in a developing system, but the consensus seems to 

 have been that there is a decrease in mitochondria during cleav- 

 age and differentiation, sometimes correlated with the parallel 

 increase of secretory granules, plastids, etc. (Meves, 1908; Guil- 

 leiTnond, 1912). However, some observations of an increase in 

 mitochondria during development were made (Romeis, 1913). A 

 study of bat development by Levi (1915) showed a constant 

 decrease in number of mitochondria during cleavage, with an 

 accompanying increase in size, until a point was reached where 

 the number of particles remained constant. 



The problem of self-duplication of particles in cells has at- 

 tracted wide interest recently, especially in connection with spec- 

 ulations on "plasmagenes" and the role of the cytoplasm in ge- 

 netic continuity of cell types. Evidence seems to point to the 

 origin of mitochondria from microsomes (Brachet, 1952; Zollin- 

 ger, 1950; Eichenberger, 1953), but not all investigators have 

 agreed that this is the only possibility (Jeener, 1952). As Lind- 

 berg and Ernster ( 1954 ) pointed out, the essential problem seems 

 to be whether self-duplication of particles is necessary for genetic 

 continuity. 



Gustafson and Lenicque (1952, 1955) stated that, in the sea 

 urchin species studied by them, the number of mitochondria is 

 low during early cleavage stages, rises to a level which is main- 

 tained for some hours, and then sharply rises again in mesen- 

 chyme blastulae and young gastrulae. Mitochondria are said by 

 these investigators to arise from smaller particles by gradual ag- 

 gregation. They interpreted these results to mean that mitochon- 

 dria cannot develop during periods of rapid cleavage, since the 

 nucleus, which controls synthetic processes in interphase, is at 

 this time concerned with processes of self-reorganization. A simi- 

 lar conclusion was reached by Agrell (1954, 1955) who made 

 mitotic counts in sea urchin embryos and found a gradient of 



