122 COMPARATIVE ANATOMY 



the longitudinal split has nothing to do with the bivalent 

 character of the U-shaped chromosomes. 



By this time the karyosome has broken up into fragments 

 which are cast out into the cytoplasm. The nuclear membrane 

 disappears, the centrosomes diverge from one another and 

 spindle-fibres are formed between them. As the external 

 spindle-fibres are formed the bivalent chromosomes appear to 

 be caught up by them and carried to the equator of the 

 spindle. In this position their bivalent character is often 

 obvious, but their actual shape is variable. They may have 

 the form of horseshoes, rings, figures-of-eight, or crosses, but 

 in each case can be seen to be made up of two parts. A 

 horseshoe is two longitudinally split curved rods joined 

 together by one end ; a ring is composed of two similar rods 

 joined together by both ends : a figure-of-eight is a twisted 

 ring, and so forth. When the spindle is fully formed the 

 bivalent chromosomes are arranged around its equator in such 

 a way that one component of each is directed towards one 

 pole, the other component towards the other pole. In the 

 metaphase the two components are simply drawn apart, so 

 that the two daughter-spermatocytes have each twelve univalent 

 chromosomes, the number having been reduced by the 

 division of each bivalent chromosome into two. This mode 

 of division is therefore very appropriately called a meiotic or 

 reducing division. As soon as it is completed the two 

 daughter-spermatocytes, each containing twelve univalent 

 longitudinally split chromosomes, prepare for a new division. 

 Two stages of the prophase are shown in fig. 25, G. The 

 longitudinal split, even if it has been obscured or has alto- 

 gether disappeared in the preceding changes, again becomes 

 very apparent in each of the twelve chromosomes. The last- 

 named are arranged round the spindle in such a manner that 

 the longitudinal split in each coincides with the equator, 

 therefore with the half of a chromosome on either side of it. 

 In the metaphase these halves are pulled apart (fig. 25, H) ; 

 the anaphase and telophase follow in the usual manner, and 

 the whole process results in the formation of four spermatids, 

 each containing half the normal number of chromosomes. 

 The spermatid is directly converted into a spermatozoon by a 

 series of changes of form and growth too complicated to be 

 described here. The process of the maturation of the ovum 



