AND OF HYBRID PIGEONS. 
37 
forms, but they are much less pronounced and more restricted 
in range. 
The abnormalities of hybrid pigeons in general may be clas¬ 
sified conveniently under three heads: (1) abnormalities in 
mitosis; (2) abnormalities in the structure of the spermatozoa; 
(3) degeneration of the germinal cells. Not all hybrids show 
these various irregularities in the same degree. All three 
classes of the phenomena just mentioned are observable in the 
sterile forms, but the fertile birds for the most part differ from 
the ordinary type only in the slightly irregular character of the 
mitosis. 
ABNORMALITIES IN MITOSIS. 
The abnormalities in mitosis are in the nature of multipolar 
spindles and asymmetrical division and distribution of the 
chromosomes (Figs. 28-39). These may exist independently 
one of another, or both may occur together in the same cell. 
They are more pronounced in sterile birds but may at times be 
seen in the fertile forms. In very many of the divisions ot 
primary spermatocytes one or the other, or both of these pheno¬ 
mena are seen. It is a curious fact that the multipolar spindles 
seem to be confined largely to the primary spermatocytes, and 
one is prompted immediately to associate the fact with the 
pseudo-reduction or formation of bivalent chromosomes which 
occurs normally at this stage of spermatogenesis. The irreg¬ 
ularities in chromatin distribution are also seen for the most 
part in the primary spermatocytes. 
Figs. 28-39 show some of the diverse forms of multipolar 
spindles that may occur. The tripolar types are by far the 
more common. Fig. 28 represents perhaps the most prevalent 
structure. It was not unusual to observe two spindles in one 
cell as shown in Fig. 29. It is very easy to see how two inde¬ 
pendent spindles could be derived through a slight modifica¬ 
tion of such types as Fig. 28, 30 or 31. When two such spindles 
exist independently in one cell, they may each bear a small 
number of the large bivalent ring form chromosomes as in Fig. 
29, or a greater number of small apparently univalent chromo¬ 
somes. In the first instance, judging from the general appear¬ 
ance of the spindles and the small number of chromosomes, it is 
