330 DEVELOPMENT OF ASCIDIANS 



Treatment with Sodium Azide Plus Malonate or Plus Selenite. 

 These experiments by which two kinds of mitochondrial enzymes 

 were simultaneously inactivated (CO. and succinodehydroge- 

 nase) were designed to ascertain whether the larvae produced 

 presented more marked caudal abnormalities. The results ob- 

 tained show that, while azide or malonate alone (respectively 

 azide or selenite) do not produce at low concentrations any ab- 

 normality in the larvae, they produce marked tail abnormalities 

 if used together ( Fig. 6 ) . 



Topographical Distribution of Mitochondria 



In "Mosaic" Eggs. The above described differential segrega- 

 tion of the mitochondria in particular cells of the developing em- 

 bryo is not an exceptional case. In mosaic eggs other interesting 

 cases have been described. The most notable is that of the Tubi- 

 fex egg. It must be recalled that first Lehmann ( 1941 ) and later 

 Carrano and Palazzo ( 1955 ) demonstrated the presence in this 

 egg, at the opposite poles, of a peculiar plasm rich in indophenol 

 oxidase or cytochrome oxidase, the "polplasma" (Fig. 7). During 

 development the polplasma is differentially distributed in the 2d 

 and 4d cells. Other research has shown that the polplasma is par- 

 ticularly rich in mitochondria, which are differentially segregated, 

 with it, in the 2d and 4d cells. A more accurate analysis (Leh- 

 mann, 1950 ) , confirmed by the use of electron and phase contrast 

 microscope (Lehmann and Wahli, 1954), has shown that larger 

 particles or mitochondria are distributed into the 4d cells, whereas 

 the smaller ones or microsomes, and only few mitochondria, are 

 segregated into the 2d cells. 



Other equally clear examples of mitochondrial segregation in 

 certain cells of the developing egg are not known, but we have 

 many indications that lead us to believe that such a process is 

 more frequent in "mosaic" eggs than one would suppose. 



It has been stated (Attardo, 1955a) that the Nadi reaction is 

 positive only in the animal pole of the uncleaved egg of Bithtjnia. 

 In the following stages of development the Nadi reaction is posi- 

 tive only in the quartets of micromeres which will give rise to 

 the ectodemi (Fig. 8). As in this case the Nadi reaction is pre- 



