. The development of the chick; an introduction to embryology. Birds -- Embryology. 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 formatio
. The development of the chick; an introduction to embryology. Birds -- Embryology. 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 polar bodies are being formed, and, when the egg nucleus is reconstituted, one. x2000. (After Har- per.) The order of stages is indicated by the letters a—g. Fig. 13.—Stages in the transformation of sperm heads into the sperm nuclei from the of them, which may be called the male pro- ovum of the pigeon, 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-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
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