514 MOLECULAR MECHANISMS OF DIFFERENTIATION 5 



Hultin, 1953a) precedes slightly the mesenchyme blastula stage. A distinct eleva- 

 tion of acetate utilization in RNA synthesis coincides with mitochondrial develop- 

 ment (Hultin, ig53d). 



In evaluating his results, Hultin (i953e,f) suggests that the more rapid incor- 

 poration of labelled amino acid into the microsome fraction in the early period of 

 development reflects the rapid production of microsomes. Similarly, an increase 

 in the rate of incorporation into the mitochondrial fraction at a somewhat later 

 stage of development was correlated to the increase in the number of mitochondria 

 mentioned before. However, it should be pointed out that coinciding with the 

 increase in mitochondrial activity there is also an increase in the incorporation of 

 amino acids into the non-centrifugable non-particulate proteins (particularly in 

 a fraction regarded as non-sedimentable protein). In attempting to ascribe to these 

 results some morphogenetic meaning, Hultin points out that the early increase in 

 microsomal activity may be brought about by a rapid multiplication of these 

 particles and that this phase is closely correlated to the onset of determination in 

 the sea urchin embryo. The later increase in the net incorporation rates of the mito- 

 chondrial and soluble fractions would be due to a corresponding increase in the 

 quantity of these fractions. This is apparently corroborated by direct counts of 

 mitochondria by Gustafson and Lenicque (1952) and by a sudden increase in the 

 enzymes usually found to be associated with initochondria as opposed to those 

 enzymes occurring freely in the cytoplasm (Gustafson and Hasselberg, 195 1). It 

 should be mentioned, however, that these claims have been questioned recently 

 (Shaver, 1955). In view of these uncertainties it should be pointed out that a 

 study of the changes in cell particulates during development of mosaic eggs 

 (Rattenbury and Berg, 1954; Berg, 1956; Reverberi, 1956) may be more reward- 

 ing and better accessible to further analysis since here particulates with distinct 

 properties become located in readily separable cells. 



These data, as they stand, indicate significant changes in the structural organi- 

 zation of the PFS during various phases of development. It is noteworthy, for 

 example, that, although the rate of protein synthesis must be very high during 

 the earliest phases of embryogenesis, the populations of neither the microsomal 

 nor mitochondrial particles seem to be present in large numbers and the constitu- 

 ents of the particulate elements, as e.g. RNA, are found mainly in the cytoplas- 

 mic matrix. Is the high rate of protein formation during this phase of develop- 

 ment due then, at least in part, to synthetic processes which occur independently 

 of these organized centers and is it possible that "templates" occur, unattached 

 to particulate elements? As the number of protein species increases during the 

 first phases of differentiation do individual particles {e.g. microsomes) become 

 diversified in their protein synthesizing "repertoire" or does synthesis of a new 

 protein require organization of a new particulate center? Is each particle respon- 

 sible for production of one or several proteins? For the elucidation of such ques- 

 tions, embryonic tissues may very well offer favorable and interesting material. 



(c) Nucleo-cytoplasmic interactions related to protein formation 



The far-reaching role which the consideration of nucleo-cytoplasmic interactions 

 has played both in experimental embryology and in physiological genetics has 



