306 Action of the Genetic Material 



Will they be henceforth self-perpetuating or does the genie material 

 continue to supply them? Does previous cytoplasmic difiFerentiation 

 play a role in any of these processes? Is there a cytoplasmic influence 

 upon the genie material, somehow controlling quantity, quahty, or 

 release of its products? 



It seems very diflBcult to derive definite answers to these and 

 similar questions from the varied and rather intricate biochemical 

 facts which have been found. The main reason is that a biochemical 

 solution for the processes of genetic determination probably requires 

 an understanding of extreme specificities paralleling the specificities 

 of the genie material; while the methods of study available allow the 

 characterization of only a single substance or a few classes of sub- 

 stances, apart from specificities recognized as serological types. Even 

 if it can be proved that these have something to do with the specific 

 differentiations, we are still ignorant of the real action of the genie 

 material. From the large body of facts, sometimes contradictory facts, 

 we may select those which, first, contribute at different points to the 

 material of the preceding discussions by endowing it with specific 

 biochemical meaning and, second, those most liable to elucidate the 

 genetic problems. 



The main fact, according to Brachet, is the existence of granules, 

 microsomes, which consist mainly of RNA and are found in a definite 

 position and relation in the cytoplasm of the egg and the embryonic 

 cells. During oogenesis RNA seems to be concerned with the synthesis 

 of "yol'^ proteins," mainly at the nuclear surface and in the cortical 

 region of the egg, near the blood supply. (All the data relate to the 

 amphibian egg if not stated otherwise.) In the unfertihzed egg the 

 RNA microsomes are arranged in a quantitative gradient which 

 remains when bilateral symmetry is established after fertilization. 

 During development the gradients remain and parallel the morpho- 

 genetic activities of the regions; quantitative measurements show 

 larger quantities of RNA in more active regions. During cleavage 

 there is no measurable RNA activity. But at the time of gastrulation, 

 simultaneously with the appearance of nucleoli, synthesis of more 

 RNA begins. At this time, also, protein synthesis starts from yolk as 

 evidenced by synthesis of alkahne phosphatase. The microsomes con- 

 sisting of RNA proteins, which are synthetized at this time, are 

 assumed to have genetic continuity because of their resemblance to 

 viruses, and are even assumed to pass from cell to cell. A main argu- 

 ment for the latter assumption is derived from experiments in which 

 development is blocked by heat shocks which are supposed to de- 



