THE REDUCING DIVISION 261 
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 
