368 PRINCIPLES OF EMBRYOLOGY 



acid (ribose nucleic acid, or RNA). The chromosomes also contain 

 protein. Much of tliis is in the form of protamines and histones, two rather 

 peculiar types of proteins which are characteristic of chromosomes and 

 hardly known elsewhere. There are undoubtedly also other types of 

 proteins in chromosomes, but there is as yet little agreement about their 

 nature (c£ Mirsky 1952). 



Figure 17.1 



Diagram of the main elements in cell structure. Inside the nuclear membrane 

 (Nm) are pairs of chromosomes ; each chromosome may contain not only 

 euchromatin, which stains deeply at mitosis, but also 'heterochromatin', 

 possibly of more than one kind (/z^, IQ ', each also possesses a centromere, or 

 spindle attachment {a) and some chromosomes form nucleoli [Nl) at specific 

 places along their length. In the cytoplasm, there may be a special granular 

 or vesicular region, the 'Golgi apparatius' (G). Most of the cytoplasm appears 

 clear, but it contains largish panicles, known as mitochondria (Afj, Mg) and 

 probably very small ultra-microscopic particles known as microsomes 



Attached to the chromosomes there may be one or more nucleoh. 

 Typically each nucleolus is formed at one definite place on a particular 

 chromosome, a so-called 'nucleolar organiser'. The number of places 

 which are active in this way probably varies in different tissues of the 

 same animal; for instance very many nucleoli may be formed at a number 

 of different places on the chromosomes of the amphibian oocyte nuclei, 



