34 
SPERMATOGENESIS OF NORMAL 
19. The smaller centrosome together with material of cyto¬ 
plasmic origin finally comes to lie inside the nuclear membrane 
and perhaps gives rise ultimately to a middle piece which be¬ 
comes obscured by a covering of chromatin, and consequently 
appears to be absent in the adult spermatozoon. 
20. The nucleus elongates to form the long head. It has 
a central core of chromatin in the form of a spiral filament 
which later splits to form a double spiral. 
21. The head, during the later stages of development, un¬ 
dergoes a very great contraction but the spiral arrangement of 
the chromatin still persists in a m.odified form, constituting a 
series of eight vesicles apparently which may possibly be eight 
univalent chromosomes. 
22. The head spine originates from a bubble-like mass of 
material which arises in the sphere. 
B. THE SPERMATOGENESIS OF HYBRID PIGEONS. 
GENERAL CONSIDERATIONS. 
It is a remarkable fact that no attempt has been made so 
far to investigate carefully the spermatogenesis or ovogenesis 
of hybrid forms. In all the mass of literature discussing or 
touching upon hybridism, so far as I have been able to ascer¬ 
tain, there has been in no instance an approach to a thorough 
study of the germ cells. Yet almost every writer states that 
through the study of hybrids, we have perhaps the best oppor¬ 
tunity for gaining a clew to many of the most vital points in the 
great problem of heredity. A number of investigators have 
remarked that in certain instances the anthers, ovary, or testes 
as the case might be, were defective, and have let the matter go 
at that. 
There is practically no divergence from the. normal type in 
the general method of procedure for the formation of sperma¬ 
tozoa in hybrids, hence it would be a needless repetition to go 
into all of the minutiae of development again. It will be taken 
for granted that the reader has determined the general plan of 
