34 



PRINCIPLES OF EMBRYOLOGY 



proceeds very far, with the production of pecuhar so-called 'lampbrush' 

 forms in which the basic chromosome threads (the chromonemata) are 

 clothed in a fluffy mass of thin hair-like projections (Fig. 2.3). Chromo- 

 somes at this stage stain very weakly in many of the dyes for which more 

 normal ones show great affinity, and in particular they are difficult to 

 stain in the Feulgen reagent which is more or less specific for desoxy- 

 ribose-nucleic acid; nevertheless it appears that they never entirely lose 



r\ 





Figure 2.3 



Structure of the loops of 'lampbrush' chromosomes from the germinal 

 vesicle of the newt oocyte. The chromonema runs horizontally across the 

 drawing. It bears small swelhngs (chromomeres) which are double. From 

 these arise loops, which are usually symmetrical about the axis of the 

 chromonema, but asymmetrical along the length of the chromonema. 

 (From Callan, unpublished.) 



their stainability in this dye, and it is probable that desoxyribose-nucleic 

 acid, which in all other circumstances appears to be an essential constituent 

 of chromosomes, is present on them throughout oogenesis also (Callan 

 1952). 



Accompanying the expansion of the chromosomes, there is also an 

 enlargement of the nucleolus, which often throws off smaller bodies so that 

 the nucleus comes to contain many nucleoli (Fig. 2.4). The chemical con- 

 stitution of these is different from that of the chromosomes; they contain 

 much ribose-nucleic acid but no desoxyribose, and also much protein 

 of a basic type involving arginine. Substances of the same general kind as 



