36 



THE DEVELOPMENT OF THE CHICK 



immediately bore through the egg-membrane and enter the ger- 

 minal disc, 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 lies 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 ^ bodieg are bei f orme d, and, 



. ' , . ... , 



when the e gg nucleus 1S reconstituted, one 

 of them, which may be called the male pro- 

 nucleus or primary sperm nucleus, moves 

 inwards and comes into contact with the 



FIG. 13. Stages in 

 the transformation of 

 sperm heads into the 

 sperm nuclei from the 

 ovum of the pigeon, 

 x2000. (After Har- 



per.) The order of nucleus (Fig. 15). The opposed faces 



stages is indicated by , ,, . . . , fl , , 



the letters a e con J u g a t l ng 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. 

 The partition apparently disappears. However, it is 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 karyokinetic way. The first 

 segmentation (or cleavage) spindle thus formed lies near the 

 center of the germinal disc 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. 



