i INTRODUCTION 3 



proceed to division. But this is by no means universally the case ; 

 in fact in several cases which have been subjected to detailed examina- 

 tion it has been proved not to be the case. 



The germ cells in this state of rapid division are termed oogonia 

 and spermatogonia respectively. Division is succeeded in both 

 cases by growth and then by a state of rest, at the end of which the 

 germs are known respectively as oocytes and spermatocytes of 

 the first order. In most cases during this period the difference 

 between the two kinds of germ becomes apparent. The spermatocytes 

 increase only slightly in size as compared with the spermatogonia 

 and undergo no diminution in number ; but some only of the oogonia 

 increase in size and become the large oocytes or unripe ova ; the 

 remaining oogonia are either absorbed as food by their successful 

 sisters, or reduced in size so as to form " follicle cells " which serve 

 as a protective covering for the oocytes. 



The follicle cells in many if not all cases contribute nourishment 

 to the oocyte, and a considerable portion of this food, termed 

 deuteroplasm or food-yolk, is precipitated in the form of globules or 

 platelets consisting chiefly of lecithin. A first stage in the storing 

 of food material in the cytoplasm appears to be the emission into the 

 cytoplasm of chromatin from the nucleus. This process has been 

 studied in detail in Echinoderm eggs by Schaxel (1911). Yolk 

 globules appear later, at first near the periphery of the egg, by the 

 transformation of this cytoplasinic chromatin. Often indeed the 

 modified cytoplasm infiltrated with chromatin is aggregated in a 

 more or less spherical body which absorbs stain, termed the yolk- 

 nucleus (Fig. 11, y.ri}; round this the yolk first appears, but after 

 the completion of yolk formation it disappears. Occasionally, as 

 Schaxel has shown to be the case in the egg of Strongylocentrotus, no 

 yolk at all is formed, the deposits of cytoplasinic chromatin con- 

 stituting the only reserve material present. Different eggs differ from 

 one another in the amount and nature of the food-yolk. The follicle 

 cells in many cases have as their final duty the secretion of an 

 outer egg-shell or chorion (Fig. 11, cli). The inner egg-shell is 

 secreted after fertilization by the cytoplasm of the egg itself and is 

 termed the vitelline membrane. 



The nucleus of the unripe ovum is in nearly every case 

 distinguished by the fact that it has the form of a vesicle con- 

 taining a clear fluid termed the nuclear sap, within which is a 

 dense mass of staining matter, different in chemical reactions from 

 chromatin, which is termed the nucleolus. Stretched across the 

 nuclear sap are a few fibres and on these the true chromatin is 

 situated as small inconspicuous grains. This arrangement of material 

 gives the nucleus of the oocyte a peculiar look which is unmistak- 

 able and which led to its being termed the germinal vesicle by the 

 older writers, whilst the nucleolus was termed the germinal spot. 



Before male and female germ cells can unite both must mature, 

 and this they do by undergoing two maturation divisions. The 



