THE GAMETES 37 



evolved animals have more yolky eggs. Thus many marine invertebrates 

 have rather little reserve, since their embryos can at a very early stage 

 obtain nutriment from the microscopic living creatures of the sea. Verte- 

 brates, in which the embryo cannot feed itself until it has developed a 

 mouth and a gut, have much more yolk; even the amphibian egg is 

 packed with yolk platelets, most fish have still more, and reptiles and birds 

 most of all, as well as extra-ovular reserves in the form of the 'white'. 

 But the rule breaks down for mammals, in which the embryo is fed 

 through the maternal placenta; although the monotremes, the most 

 primitive representatives of the mammal stock, have eggs nearly as yolky 

 as reptiles, in the true mammals the ovum contains scarcely any reserve. 

 Most insect eggs, on the other hand, have a great deal of yolk, which is 

 accumulated towards the middle instead of at one end (these are spoken 

 of as centrolecithal eggs). 



3. Follicles and membranes 



During growth within the ovary, the cortex of the egg is usually 

 closely invested by a layer of so-called 'follicle cells' which, presumably, 

 play a major part in transmitting the materials for the growing oocyte; 

 they may also be the main determinants of the cortical structure, although 

 this is not definitely known. In mammals, the layer of follicle cells becomes 

 very tliick ; in fact they increase to a largish spherical mass, within which 

 a secretion is formed which hollows out the mass until the oocyte is 

 hanging from a sort of stalk. This secretion contains the 'follicular hor- 

 mone' which produces oestrus in the female mammal. When the egg is 

 ripe, the follicle bursts and the egg, still surrounded by a layer of follicle 

 cells, is set free to reach the Fallopian tubes and thus travel down to the 

 uterus ; meanwhile the remains of the follicle forms the 'yellow body' or 

 corpus luteum, from which is secreted the luteal hormone, an important 

 factor in pregnancy. 



In other animals, the follicle cells are less in evidence, although probably 

 always present. In insects the eggs are arranged in strings in the ovary; 

 and there may be no special nutritive cells, or a group between each egg, 

 or a single group at the end of each string wdth projecting strands leading 

 down to the growing eggs (Fig. 2.5). The follicle cells or 'nurse' cells, 

 as they are also called, are themselves often the site of active synthesis. 

 In Drosophila and other insects their nuclei are polyploid, the chromo- 

 somes having divided frequently without any accompanying division of 

 the cytoplasm (Painter and Reindorp 1939) ; this phenomenon is often 

 found in secretory or synthetic cells in insects. In most cases the sub- 

 stances formed in the nurse-cells are passed almost completely into the 



