56 GROWTH AND MORPHOGENESIS 



ing of the unfertilized egg is not sufficient to produce partheno- 

 genetic development. A "second factor" is required. In Bataillon's 

 experiments, the second factor was the introduction of a nucleate 

 cell into the egg. An aster forms after a while around the injected 

 cell, and the presence of this second aster is of course necessary in 

 order to obtain mitotic cleavage. The experiments of Shaver (1953) 

 have shown that the nucleate cell can be replaced by cytoplasmic 

 granules (mitochondria and microsomes) extracted from a variety 

 of cells (testis, red blood cells, frog embryos at stages later than the 

 blastula, etc.). The fact that these granules are inactivated by ribo- 

 nuclease strongly suggests that RNA is somehow involved in the 

 activity of the granules and thus in the formation of the asters. 



Experiments with ribonuclease have also been carried out in the 

 case of egg cleavage and have shown that RNA is definitely involved 

 in mitosis and, therefore, in growth processes. It was first found by 

 Thomas et al. (1946) that pricking fertilized eggs with a needle 

 which had been dipped in a solution containing ribonuclease stops 

 cleavage. More careful experiments, which involved the micro- 

 injection of known amounts of the enzyme into one of the blasto- 

 meres at the 2-4-cell stage, were performed by Ledoux etal.{\95S). 

 Finally, it was observed by Brachet and Ledoux (1955) that ribo- 

 nuclease easily penetrates into cleaving eggs as into so many other 

 cells (see Chapter 1). All one has to do is to place a morula in a 

 ribonuclease-containing solution and observe the subsequent 

 cleavages. As shown in Fig. 18 (p. 61), mitosis is completely arrested 

 in the outer blastomeres. The swollen nuclei remain in the inter- 

 phase and it is exceptional for destruction of an already existing 

 mitotic apparatus to be found. It has been observed by Chevre- 

 mont and Chevremont-Comhaire (1955) that ribonuclease exerts a 

 similar effect in tissue cultures: cell division quickly stops in fibro- 

 blasts with the nuclei again blocked in interphase. An interesting 

 fact, observed by Chevremont et al. (1956), is that DNA synthesis is 

 not interrupted in these blocked nuclei; their DNA content slowly 

 reaches 4 times the value found for the spermatozoon, but no mitosis 

 follows. It is not known whether the same situation also occurs in 

 amphibian eggs, since measurements of the DNA content of the 



