36 



THE DEVELOPMENT OF THE CHICK 



atozoa immediately bore through the egg-membrane and enter the 

 germinal disc, within which the heads, which represent the nuclei 

 of the spermatozoa, enlarge and become transformed into sperm - 

 nuclei (Fig. 13). In the hen's egg five or six usually enter. The 

 fate of the middle piece and tail of the spermatozoa 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 spindle 

 is in process of formation ; it lies in the center 

 of a group of granules at the surface of the 

 egg, which is bounded by a non-granular 

 zone of protoplasm, called by Harper the 

 polar ring, in which the sperm-nuclei ac- 

 cumulate. 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, 

 when the egg nucleus is reconstituted, one 

 of them, which may be called the male pro- 

 nucleus or primary sperm nucleus, moves 

 inwards and comes into contact with the 

 egg nucleus (Fig. 15). The opposed faces 

 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. 

 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-nucieus 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. 



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 

 stages is indicated by 

 the letters a — g. 



