AND OF HYBRID PIGEONS. 
29 
nucleus. A vacuole-like area persists in the sphere, marking 
the spot where the centrosome lay (Fig. 18, hs.). The two new 
centrosomes move apart for a short distance, but remain con¬ 
nected by a barrel-shaped sheath or mass of material (Fig. 
18, c.). In the. meantime the nucleus gradually approaches that 
part of the cell wall furthest removed from the centrosomes. 
The next noticeable change is the disappearance of the 
Barrel-shaped connective and the existence in its place 
of a very delicate fibril uniting the two centrosomes. 
One centrosome has enlarged, moreover, and if favor¬ 
ably situated for observation, can be seen to be a complete ring 
(Fig. 19). The connecting fiber just mentioned grows rapidly 
and is soon visible as a thread passing from the smaller centro- 
sonie back through the ring which it touches at one side, and 
continuing finally outside the cell (Figs. 20, 21). The 
vacuole or bubble-like area of the sphere gradually moves 
around the nuclear periphery in the meantime until it 
comes to lie at the pole opposite the centrosomes (Figs. 19- 
21, hs.). The centrosomes next approach the nucleus and so 
orient themselves that the smaller one points toward it. As 
they draw nearer a slight invagination appears in the nuclear 
wall and the smaller centrosome moves into it (Fig. 21, c.). 
The smaller centrosome is seen finally to lie inside the. nucleus. 
The ring becomes closely applied to the nuclear wall where 
the invagination occurred. The filament or axial fiber of the 
tail, as it will ultimately be, continues to elongate. 
A close examination of this stage discloses the fact that the 
invading centrosome has seemingly carried with it into the 
nucleus an enveloping mass of extra-nuclear material which, 
judging from its reaction to stains, is of the same nature as the 
sphere substance. Taking this into account together with the 
fact that in the adult spermatozoon there is no visible middle 
piece, it seems probable that this internal centrosome together 
with its surrounding mass of cytoplasm may perhaps later de¬ 
velop into what corresponds to the middle piece of other sper¬ 
matozoa. But here, since the material lies within the. nucleus, 
it is obscured by the rearrangement and crowding back of the 
chromatin mass when the long head begins to form. Nothing 
