EIGHT FIGURESThe clianges in the medullary bone of laying birds offer unusual opportunities for the study of several aspects of bone formation and destruction. Thus the rapid cyclic formation and breakdown of this bone in laying pigeons enabled Bloom, Bloom, and McLeaii ('41) to conclude that the various types of cells of bone are temporary functional states of the same cell. We have continued to study tlie medullary bone of laying birds and have tried to find out whether the cellular transformations are especially rapid in the chicken with her large clutches and extended periods of egg laying.

No abstract

The purpose was to determine the sites, times and frequency of mitotic activity in primordial germ cells in the white Leghorn chick embryo during the period of migration. Colchicine was employed to facilitate the identification of dividing germ cells in embryos ranging in age from 18 hours to five days of incubation (stages 3-27). An increase in the number of germ cells was observed during the period of migration, due primarily to proliferation of intraembryonic cells, since no significant increase in the number of extra-embryonic germ cells was seen during this period. The number of germ cells during this period ranged from 43 at 18 hours to 2211 at 120 hours. Two periods of intense proliferation were observed, the first between 48 and 72, the second between 96 and 120 hours. This coincided with a simultaneous increase in the number of germ cells during these periods. Dividing germ cells were present in the extraembryonic blood vessels anterior, lateral and posterior to the embryo at 28, 48 and 72 hours and within the intra-embryonic circulatory network at 48, 72, 96 and 120 hours. At 72, 96 and 120 hours, dividing germ cells were numerous in the tissues of the dorsal mesentery adjacent to the developing gonads and within the gonads. Dividing germ cells were also located in head mesenchyme, limb buds and mesenchyme surrounding the notochord-neural tube complex. Dividing germ cells were found in the chick embryo throughout the entire migratory period.

The germinal crescent in the chirk embryo is characterized by small, PAS-positive, nonglycogen granules from 1.5 to 5 4 in diameter. The primordial germ rells (PGCs) were found to originate in and separate from the germinal crescent endoderm through stage 7 (2 somites). Shortly after separation most of the granules in the PGCs lost their organization and the PAS-positive material was distributed irregularly throughout the cytoplasm. A few of these granules remained within the cells indefinitely. Glycogen of an agranular nature which had shifted to one pole of the cell was observed a t stage four. Granular glycogen which was distrihuted throughout the rytoplasm was not observed prior to stage 7 or 8.Cell counts on individual embryos showed noticeable variations as to the number of germ cells between embryos of the same stage. For example, in stage 4 embryos the minimum number of cells rounted, including attached and free, was 78 and the maximum 169, while in stage 9 the minimum was 83 and the maximum 469 cells. After separation the germ cells were observed almost anywhere between the ectoderm and the endoderm although the majority remained in the area whew they originated.Swift ('14) concluded that the primordial germ cells (PGCs) were derived from the germ wall endoderm, a t its junction with the area pellucida, anterior and antero-lateral to the forming embryo. Because of its shape, this area became known as the germinal crescent and the germ cells were found t o originate in i t during the primitive streak and up through the three somite stage. These cells then entered the forming blood vessels and with the establishment of circulation were carried to the gonadal site. These observations were confirmed by Goldsmith ('28) in the chick and by Blocker ('33) in the English sparrow.Further proof of the area of origin of the PGCs in the chick was provided by the experiments of Goldsmith ('35) and Simon ('57a) in which sterile embryos resulted from removal of the germinal crescent.The in vitro experiments of Simon ('57b) demonstrated the role that the circulation plays in the migration of the germ cells. This investigator placed a blastodisc, from which the germinal crescent had been re-AM. J. ANAT., 185: 87-112.