36 PRINCIPLES OF EMBRYOLOGY 



have extra heterochromatin (in the form of a supernumerary Y chromo- 

 some) but this has been disputed by Callan (1948), and it is not clear what 

 relation exists, if any, between the cytoplasmic and nucleolar ribonucleic 

 acid on the one hand and the heterochromatic parts of the chromosomes 

 on the other. 



In spite of the ribonucleic acid materials apparently given off from the 

 nucleus, it is probable that the latter is not the main site of synthesis in 

 the oocyte, hi fact, Brachet (1952I)) has argued that the nucleus contains 

 less than its due share of the cellular enzymes, and, for instance, accounts 

 for only a very small fraction of the respiration of the cell; this result, 

 however, was probably a consequence of inadequate methods of cultiva- 

 ting the isolated amphibian germinal vesicle with which he worked, and 

 as far as respiration is concerned, the nucleus is probably just as active as 

 the cytoplasm (Callan 1952). 



It is in the latter, however, that the major synthesis of the food reserves 

 takes place. In many organisms there appears in the cytoplasm of the 

 young oocyte a body which has received the name of the 'yolk nucleus'. 

 It is particularly well seen in the eggs of some spiders, in which it has a 

 laminated structure and is birefringent. In Amphibia it is represented in 

 the young oocyte by a small mass in the cytoplasm which later expands 

 and disperses to form a peripheral sheet lying just below the outer surface 

 of the cell. It is usually considered to be constituted of mitochondria, and 

 histochemical tests reveal the presence in it of fats, including the pecuhar 

 phosphatides which give the so-called plasmal reaction, and of several 

 enzymes such as indophenoloxidase, dipeptidase, etc. It appears almost 

 certain that the yolk nucleus is the seat of a particularly active synthesis 

 of fats and proteins. 



The reserve foodstuffs in eggs are often collectively referred to as 'yolk'. 

 But this is really a loose use of the word. Strictly speaking, the true yolk 

 is only one part of the reserve; it consists of platelets or lumps of a protein- 

 lipoid substance. Besides it, the reserve contains globules of more or less 

 liquid fat, and granules of carbohydrate, which are usually in the form of 

 glycogen. In ova which contain fairly small quantities of reserves, these 

 materials may be scattered more or less evenly throughout the cytoplasm ; 

 such eggs are known as 'ohgolecithal', meaning that they contain little 

 yolk. In most eggs with more than a very small quantity of reserve 

 material, this is accumulated towards one end, the heavy vegetal pole 

 referred to above; such eggs are called 'telolecithal'. There is a whole 

 range of them, from only moderately yolky forms to bird or teleost eggs, 

 in which the enormous mass of reserve food almost swamps the tiny 

 patch of living cytoplasm. As a very rough general rule, the more highly 



