382 PRINCIPLES OF EMBRYOLOGY 



takes place, never occurs in the absence of desoxyribose nucleic acid. In 

 the synthesis of cytoplasmic proteins, it is ribose nucleic acid which is 

 thought to play the active part. Chemical analyses always demonstrate 

 an unusually high concentration of RNA in rapidly growing cells or in 

 those which are actively secreting protein (e.g. some glands, hair-forming 

 cells, etc.). This RNA is largely located in the cytoplasm, but there is 

 often also a considerable enlargement of the nucleoli, which are rich in 

 RNA. For example, nucleoh are absent in the cleavage cells of the amphi- 

 bian neurula, but appear at about the time of gastrulation, when there is 

 evidence that specific proteins begin to be produced. 



There are at present two main theories about the processes of protein 

 synthesis in embryonic or developing cells. The first is that of Caspersson 

 (Reviews : 1947, 1950). It is based chiefly on studies which use spectroscopic 

 methods to take advantage of the fact that the purine and pyramidine 

 bases incorporated in the nucleic acid molecule have very characteristic 

 absorptions in the ultra-violet; this makes it possible for the nucleic acids 

 to be identified within living cells, although it must be pointed out that 

 there are considerable technical difficulties and the method has come in 

 for a good deal of criticism. From his observations, however, Caspersson 

 has come to the following conclusions. The euchromatin, which consists 

 largely of histone and DNA, synthesises replicas of itself and also produces 

 other complex proteins, which could be the agents through which gene- 

 action is exerted: however, the spectroscopic evidence does not suffice 

 to suggest much about them. The heterochromatin (or, more generally, 

 the 'nucleolus-associated chromatin') which contains an important pro- 

 portion of RNA, is supposed to control the nucleic acid metabolism of 

 the whole cell. It also produces proteins, which tend to be rich in di- 

 amino-acids. These accumulate in the nucleolus, and diffuse from there to 

 the nuclear membrane. On the outside of this, an intensive production of 

 RNA-protein takes place and this is the main source of the cytoplasmic 

 proteins, which are thus supposed to be formed in the immediate neigh- 

 bourhood of the nucleus (Fig. 17.5). 



The other main theory is that of Brachet (cf. 1950, 1952). It differs from 

 that of Caspersson in being derived from a large variety of biochemical, 

 cytological and embryological investigations rather than from a single 

 technical method such as spectroscopy. In its conclusions it lays much 

 more stress on the role of the various types of cytoplasmic particle in 

 protein synthesis. Brachet supposes indeed that the ribose nucleotides 

 formed by the nucleolus (or heterochromatin) do not merely combine 

 with proteins in the neighbourhood of the nuclear membrane, but take 

 part in the formation of microsomes scattered throughout the whole 



