212 PRINCIPLES OF EMBRYOLOGY 



in microscopic preparations. These he called microsomes. They can be 

 shown, both by direct chemical methods and by their staining reactions, 

 to be ribonucleo-protein in constitution. They have in general rather low 

 enzymatic activity, but Brachet suggests that they may be the sites at 

 which protein is synthesised. It is worthy of remark that according to a 

 recent report, ribonucleic acid itself shows enzyme activity and is capable 

 of breaking down dipeptides (Binkley 195 1); such enzyme actions are 

 frequently reversible and it may be that the nucleotide moiety of the 

 microsomes plays a direct role in coupling together amino-acids to form 

 proteins. The granules certainly increase greatly in number at the begin- 

 ning of gastrulation, particularly in the organiser region and in the 

 invaginating mesoderm; a httle later they appear in large numbers in the 

 developing neural plate. Brachet marshals a large body of evidence, 

 which is in sum quite impressive although unfortunately most of it is 

 somewhat indirect, in support of the hypothesis that these particles play an 

 essential role in induction. For instance, if gastrulae are given a high 

 temperature shock (about 36-37° C, i.e. some 2° C. below the lethal 

 temperature) the gastrulation stops completely and no induction occurs; 

 simultaneously the microsomes lose their nucleic acid. A very similar 

 state of affairs occurs in hybrids between certain species of frogs, in which 

 the disharmony between the cytoplasm and nucleus leads to a blocking 

 of development at the gastrula stage; this evidence shows that the genes 

 must be involved in the metabolism of the microsomes, a point which is 

 of great general importance, as we shall see later (p. 3 82). In both the heat- 

 treated and the hybrid gastrulae the rate of oxygen consumption ceases 

 to rise from the time at which development stops. [Barthand Sze (1951) 

 have shown that the summed oxygen uptake of a piece of organiser and a 

 piece of gastrula ectoderm is higher when they are placed together so that 

 an induction can occur than it is when they are kept separate. But it is 

 not clear whether the extra oxygen is used for the actual evocation itself, 

 or for the neural differentiation wliich the induced ectoderm under- 

 goes.] It is rather surprising to fmd that these changes can often be re- 

 versed and development be started up again if a fragment is transplanted 

 to a normal host embryo, which must be able to supply some essential 

 substance lacking from the blocked tissues. The nature of the substance 

 is obscure. 



Again, Holtfreter (1945, 1948/)) showed that when gastrula ectoderm 

 is submitted to abnormal media (producing so-called sub-lethal cytolosis) 

 there is a massive appearance of microsome-Hke bodies in the cytoplasm. 

 Waddington and Goodhart (1949) made much the same observation when 

 studying the mode of action of the steroid-hke evocators. The location 



