II MEIOSIS IN THE FEMALE 57 



thus a set of four cells derived from the one oocyte L Cases where the 

 first polar body does thus divide are very common, but the division does 

 not always occur, simpty because the degeneration of the polar body as a 

 cell has often gone too far. Stages in its loss of power to divide can be 

 observed. In the sponge Sycon (Jorgensen, 1910 a), for example, the first 

 polar body makes a beginning of a mitotic division which, however, 

 generally remains uncompleted. 



As a rule, the polar bodies disintegrate and disappear soon 

 after the e^g has been fertilized, but sometimes they can be traced, 

 adhering to the outside of the developing embryo, for a long time 

 (A scaris) . 



An interesting confirmation of the essential homology l^etween the 

 polar bodies and the ripe c^g is provided by an observation of Lefevre 

 (1907) on Thalassema. The egg of this annelid, like that of so many 

 others, can be induced to develop without fertilization if placed in a 

 suitable excitant chemical medium (see Chapter III.). In some cases 

 not only was the egg induced by this means to start cleavage, but the 

 polar bodies also divided repeatedly, producing a morula-like cluster 

 of minute cells — in some instances as many as sixteen. 



The later stages of oogenesis — i.e. the two actual mitoses of the 

 meiotic phase — are illustrated in Figs. 11, 32, 33, 82. As in the case of 

 the male meiosis, the important stages are to be sought in the long-drawn- 

 out prophase of the meiotic mitosis itself. The early stages of this 

 process in the cat are illustrated in Fig. 21. 



It will be seen that up to the diplotene stage (F) the process is closely 

 parallel to the corresponding stages in spermatogenesis. Instead, 

 however, of straightway condensing into the definitive chromosomes of 

 the first meiotic division, the bivalents now lose their sharp contours, 

 diminish in staining capacity and become distributed in an irregular 

 fashion through the nucleus, while at the same time the nucleolus, 

 which was already present in the young oocyte, increases in size. Thus 

 the nucleus passes into the germinal vesicle condition. 



The manner in which the bivalents pass into the germinal vesicle and 

 re-emerge as the definitive chromosomes of the first meiotic division in 

 the dog-fish is shown in more detail in Fig. 22 (Marechal, 1907). There 

 is a typical pachytene bouquet (A), the orientation of which is lost in the 

 diplotene stage (B). The chromosomes now lose their sharp outlines by 

 reason of the development of very numerous thread-like outgrowths 

 (Figs. 22, D; 23). These being arranged more or less at right angles to the 

 central axis of the chromosome give the whole structure a characteristic 

 appearance which has often been compared to that of a cylindrical chimney 

 brush. Synchronously with the development of these outgrowths the 

 chromosomes as a whole lose their characteristic chromatin staining 



