88 GROWTH AND MORPHOGENESIS 



would be kept to a minimum and the dividing nucleus might thus 

 be considered as inactive in so far as the control of anabolic activities 

 is concerned. In other words, during cleavage, the egg would not be 

 very different from an enucleate egg. In fact, it is well known that 

 sea urchin and amphibian eggs can cleave repeatedly in the com- 

 plete absence of the nucleus (see Brachet, 1957, 1960 for a much 

 more complete discussion of this problem). 



At gastrulation, mitotic activity slows down considerably and 

 the nuclei now "have time" to build up nucleoli during interphase. 

 Obviously, synthesis of RNA, as well as that of DNA, must occur 

 in the nuclei from that stage on. Very recent observations from our 

 co-workers Bieliavsky and Tencer (1959) substantiate this expec- 

 tation. Working with dissociated cells from amphibian blastulae, 

 gastrulae and neurulae, and using incorporation of tritium-labeled 

 uridine and autoradiography as the main tools, they found that 

 this RNA precursor is exclusively incorporated into DNA during 

 cleavage. The eggs are thus capable of utilizing this ribose deriva- 

 tive for DNA synthesis. At gastrulation and afterwards, the labeled 

 uridine is incorporated in both DNA and RNA. In the case of the 

 latter, labeling is stronger in the nuclei than in the cytoplasm. 



The view that intricate interactions between the nucleus and the 

 cytoplasm are required in order to obtain morphogenesis at stages 

 following cleavage, is also supported by the fact that in amphib- 

 ians many hybrid combinations become lethal at the gastrula 

 stage. Despite the presence of an abnormal sperm nucleus, cleavage 

 is perfectly normal. But development stops at the young gastrula 

 stage unless a fragment of the lethal embryo is grafted into a normal 

 gastrula. As pointed out before, such a graft is followed by a "revi- 

 talization" of the fragment, in which morphogenesis and RNA 

 synthesis are, as usual, closely linked together. Obviously, the pres- 

 ence of the abnormal nucleus does not permit the synthesis of 

 certain substances which are required for morphogenesis. These 

 substances, which are not species-specific, are supplied by the nor- 

 mal host in the case of a graft, if followed by revitalization. Nothing 

 is known about the nature of these substances, but it might be 

 significant that in the lethal hybrids of both Anurans (Brachet, 



