26o 



PRINCIPLES OF EMBRYOLOGY 



with the cells nearby, would seem to provide a good opportunity for 

 studying the processes of protein synthesis in embryos. Some histo- 

 chemical work on the growth of wool has been pubhshed by Hardy (1952), 

 who finds that, as usual, RNA is present in high concentration in the cells 

 in which growth is occurring most rapidly. The distribution of some 

 other substances thought to be important for protein synthesis, such as 



ssss ssss 

 Birth 



Figure 12.4 



The rate of growth of the wool fibre from a follicle depends on (i) the 

 amount of available substrate, which is proportional to the area of skin, (2) 

 the efficiency of the follicle divided by the sum of the efficiencies of all the 

 other follicles with which it is competing. In the figure on the left, the time 

 of initiation of the primary [Pc and Pi), and secondary (S) follicles is shown 

 below. The dotted line shows the increase in skin area. The group of dashed 

 lines show various relationships between the efficiency of a follicle and its 

 time of initiation — the upper curve being that characteristic of a coarse 

 fleece, the lower one of a fine fleece. The stepped line shows the increase in 

 the total efficiencies of all the follicles. The figure on the right shows the 

 growth curves of a series of fibres calculated on this basis (medium fleece). 

 (After Fraser 1952.) 



sulphydryl-containing proteins, is also interesting, but the whole range of 

 data cannot yet be fully interpreted (Fig. 12.5). Investigations by Lees and 

 Picken on the influence of genetic factors on the rate of synthesis and the 

 kind of protein produced in Drosophila hairs are referred to on p. 337. 



5. The trunk: endodermal structures 



It is, of course, the endoderm which forms the innermost structures, 

 namely the gut and its annexes. In fact it produces only the central core of 

 these, since the adult organs include an investment of mesoderm which 

 clothes the original endodermal rudiments. 



We have seen that in the Amphibia the process of gastrulation itself 



