146 ZOOLOGY 



probably of mitochondrial nature), were under suspicion as pro- 

 ducers or auxiliaries in vitellogenesis. This, of course, is rightly 

 so, but the whole case is vitiated by the fact that certain ones only 

 are picked out in certain cases as the sole agents. The cases of 

 Ascaris and Patella appear to be mutually incompatible. 



Reference has been made above to the work of Hogben on the 

 role of the nucleolus, and as a very important suggestion was made 

 by him at that time, the outlines of his results must be briefly 

 summarised (55-57). 



Despite individual differences between Periplaneta and Libel- 

 lula, it appeared that the nucleolus became differentiated into two 

 phases. In Periplaneta these phases follow one after the other, 

 in Libellula they occur simultaneously. From one phase (the 

 first in Periplaneta, the cortical in Libellula) no permanent struc- 

 tures are derived, but from the other phase, vacuoles are emitted 

 either many at a time (Periplaneta) or one only at a time (Libel- 

 lula). These vacuoles pass out into the cytoplasm and there turn 

 into yolk (deutoplasm). In Libellula they are often surrounded 

 by mitochondria derived from the so-called yolk nucleus, and 

 Hogben is led to suggest that the formation of yolk is due to the 

 intricate interaction of the metabolic functions of plasmosome 

 (active nucleolus), mitochondria and Golgi apparatus. This sugges- 

 tion is of considerable value and appears to have been lost sight of 

 by everyone until a reference is made to it by Harvey in 1929 (46). 

 It will be seen from the summary of work on oogenesis set out 

 below (pp. 148 et seq.) that no attempt has been made to follow up 

 this suggestion and to test whether the facts can be interpreted on 

 this basis. 



Starting with the vertebrates, we know that the yolk contains 

 proteins, fats and lipins, but in the invertebrates, although this is 

 still true, there are other reserve materials present as distinct 

 granules or spheres in the cytoplasm. These various substances 

 Brambell considers are elaborated in different ways either directly 

 or indirectly from the Golgi apparatus, or from mitochondria, 

 from nucleolar material or again independently from the ground 

 cytoplasm. He therefore considered that the term yolk (using the 

 term generally for reserve materials) should always be qualified 



