G. REVERBERI 323 



those chemical chain reactions that bring about the processes of 

 differentiation. 



One of the best examples of differential segregation of the 

 mitochondria in certain cells of the developing embryo is offered 

 by the ascidian egg (Reverberi, 1956a). Janus green staining 

 reveals in this egg the presence of a large quantity of mitochon- 

 dria. In the unfertilized egg they are spread' everywhere in the 

 cytoplasm, except at the animal pole, where there is the nuclear 

 sap of the collapsed nuclear vesicle. Between the two zones runs 

 a circular, sinuous line of large granules of an intense green 

 (Fig. 1, a). Upon centrifuging the unfertilized gi^een egg, the 

 colored mitochondria gather between the animal hyaline cap and 

 the large mass of deutoplasm and pigments. 



The local segregation of the mitochondria starts at fertilization. 

 The uniformly colored egg in a few minutes becomes clear — only 

 the vegetal pole, to which migrates the mass of green-colored 

 mitochondria, becomes green (Fig. 1, b, c, and d). With a new 

 migration the mitochondrial mass reaches the dorsal part of the 

 egg, where normally the yellow crescent is found (Fig. 1, e). The 

 first three segmentations of the egg segregate the mitochondria 

 in the two posterior vegetal blastomeres (Fig. 1, / and g). 

 These blastomeres are the representatives of the mesodermic 

 organs. With the next egg divisions the mitochondria are segre- 

 gated only in the cells of the muscular line (Fig. 1, h, i, j, and k). 

 The tadpoles originating from eggs treated with the Janus green 

 at the beginning of development have green colored tails; more 

 exactly, the green coloration is localized in the muscular cells of 

 the tail (Fig. 1,1). 



The very impressive movements of migration and segregation 

 of the mitochondria just described and the apparent existence of 

 an accurate mechanism controlling these movements raise the 

 question of the meaning of such processes. 



It is necessary, however, to remove at once the idea, possibly 

 raised by the above described processes, that the other blas- 

 tomeres of the developing embryo are deprived of mitochondria. 

 First of all, in the ascidian egg there are probably at least two 

 kinds of mitochondria. Meves (1913), Duesberg (1926), Conklin 



