36 THE DEVELOPMENT OF THE CHICK 
immediately bore through the egg-membrane and enter the ger- 
minal dise, within which the heads, which represent the nuclei of 
the spermatozoa, enlarge and become transformed into sperm nuclei 
(Fig. 13). The fate of the middle piece and tail of the sperma- 
tozoa is not known in birds, but it is improbable that they furnish 
any definitive morphological element of the 
fertilized egg. At the time of entrance of 
the spermatozoa the first maturation spin- 
dle is in process of formation; it hes in the 
center of a group of granules at the sur- 
face of the egg, which is bounded by a 
non-granular zone of protoplasm, called by 
Harper the polar ring, in which the sperm- 
nuclei accumulate. External to the polar 
ring the protoplasm is granular again (Fig. 
14). 
The sperm-nuclei remain quiescent while 
the polar bodies are being formed, and, 
Spemnine icnmeene when the egg nucleus is reconstituted, one 
sperm nuclei fromthe Of them, which may be called the male pro- 
ovum of the pigeon. nucleus or primary sperm nucleus, moves 
x 2000. (After Har- inwards and comes into contact with the 
per.) The order of Goo nucleus (Fig. 15). The opposed faces 
stages is indicated by oa . : p 
fhe letrerala as of the conjugating nuclei become flattened 
together, until the contours form a single 
sphere, the first segmentation nucleus, in which a partition sep- 
arates the original components, viz., the sperm and egg nucleus. 
Fig. 138.—Stages in 
the transformation of 
The partition apparently disappears. However, it 1s very un- 
likely that a complete intermingling of the contents of the two 
germ-nuclei takes place, because in other groups of animals where 
the processes have been more fully studied, it has been determined 
that each germ-nucleus forms an independent group of chromo- 
somes of the same number in each. 
Shortly after its formation, the first segmentation nucleus 
prepares for division in the usual karyokinetiec way. The first 
segmentation (or cleavage) spindle thus formed lies near the 
center of the germinal dise a short distance beneath the surface 
and its axis is tangential to the surface, or, in other words, at 
right angles to the axis of the egg. The fertilization may be 
considered to be completed at this stage. 
