CHAP. II 



MEIOSIS 29 



chromosomes. Hence the first mitosis of the meiotic phase — i.e. the 

 meiotic division proper — is often known as the heterotype, and the second 

 one as the homotype, division. Before proceeding to a detailed considera- 

 tion of this important part of the Ufe-cycle it will be necessary to give a 

 brief sketch of the course of gametogenesis. 



The natural starting-point for this sketch is the primordial germ cell. 

 This is of course one of the products of those divisions of the unicellular 

 stage of the zygote by which it becomes transformed into the multi- 

 cellular adult, and it may be defined as the first of the mass of cells thus 

 produced to be dedicated to the formation of reproductive cells alone. 

 It is only in a few cases that this cell has actually been identified in the 

 developing embryo (see Chapter III.), but whether visibly recognizable 

 or not it is plain that such a cell (or cells) must occur in the development 

 of every organism. 



In some cases, for example in Ascaris megalocephala (p. 80), the 

 nuclei and the character of the mitoses of the primordial germ cell and 

 its derivatives are distinguishable from those of the other tissues of the 

 organism by various features, but in most cases they are essentially 

 similar until the meiotic phase is ushered in by the prophase of the 

 penultimate division before the formation of the gamete. 



The general course of gametogenesis is very similar in the two sexes, 

 differences in detail being associated with the relatively enormous size 

 of the macrogamete. Correlated with this, only one of the four cells 

 resulting from the two divisions of the meiotic phase becomes, in the 

 female, a functional gamete, the other three forming the very minute 

 " polar bodies." 



A diagrammatic scheme of the course of gametogenesis of both sexes 

 is given in the accompanying diagram (Fig. 13). A few words of general 

 description will suffice, as cytological details will be given in actual cases. 



The period of multiplication reaUy involves a much greater number of 

 divisions than shown in the diagram. The cells in this period are known 

 as spermatogonia and oogonia (or ovogonia). In some animals, the earUer 

 generations of these cells differ in certain visible characteristics from the 

 later ones, with the result that the former are often termed primary 

 spermatogonia (or oogonia) and the latter secondary. 



The cells in which the first meiotic division occurs in the male are the 

 primary spermatocytes. This division gives rise to the secondary sperma- 

 tocytes. For the sake of brevity, these cells may be referred to as 

 Spermatocyte I. (primary) and Spermatocyte II. (secondary), etc. Simi- 

 larly, the two divisions of the meiotic phase may be referred to as 

 Meiosis I., Meiosis II., and the various stages of the two mitoses as 

 Prophase I., Metaphase I., Anaphase II., etc., according as they belong 

 to the one or the other of the two divisions. 



