244 RECENT CYTOLOGY 



enormously exaggerated relatively to the size of the 

 ovum itself. 



The original tetrad is believed in all cases, and has 

 been actually observed in a few cases, to arise by a 

 separation of the two fused chromosomes, followed by 

 a division of each of these bodies into two. In cases 

 where the chromosomes retain their rod-like appear- 

 ance throughout these changes there would seem to be 

 some doubt as to whether the first of the divisions 

 giving rise to the ' tetrad ' is transverse or longitudinal 

 in direction, and it is possible that the process may be 

 different in different cases. But it is generally agreed 

 that the first division separates the two original 

 chromosomes, and that at the first of the two nuclear 

 divisions which ensue the members of a pair of parental 

 chromosomes pass into separate nuclei. The second 

 division, on the other hand, like an ordinary mitosis, 

 separates halves of chromosomes. This agreement 

 among authorities is explained by the circumstance 

 that those observers who have seen a longitudinal 

 first division believe that the parental chromosomes 

 conjugated side by side, whilst those who describe a 

 transverse division describe also an end-to-end con- 

 jugation of the chromosomes. 



The first of these two ideas is the one illustrated in 

 the accompanying diagram (Fig. 43), representing the 

 behaviour of a single pair of parental chromosomes 

 during the two nuclear divisions which give rise to 

 four sperm cells. The chromosome derived from one 

 parent is shaded, whilst the other is left blank. 



Thus the first of the two gamete-producing divisions 



