. Analysis of development. Embryology; Embryology. 176 Embryogenesis: Preparatory Phases those of others, as definitive proof of the ex- tragonadal origin of all of the germ cells in the chick. He points to three somewhat anomalous results: (1) Sterile gonads may be obtained from grafts that contain the germinal crescent as well as from those in which that area has been excluded. (2) Sterile gonads may also form from grafts of the gonad-forming area taken at a stage when it contains germ cells. (3) Germ cells Dorsal. Ventral Fig. 52. Diagram illustrating the location of the presumptive primord
. Analysis of development. Embryology; Embryology. 176 Embryogenesis: Preparatory Phases those of others, as definitive proof of the ex- tragonadal origin of all of the germ cells in the chick. He points to three somewhat anomalous results: (1) Sterile gonads may be obtained from grafts that contain the germinal crescent as well as from those in which that area has been excluded. (2) Sterile gonads may also form from grafts of the gonad-forming area taken at a stage when it contains germ cells. (3) Germ cells Dorsal. Ventral Fig. 52. Diagram illustrating the location of the presumptive primordial germ cells in urodeles at the early yolk plug stage according to experiments of Nieuwkoop ('46). Dorsal lip of blastopore; P, area of presumptive lateral plate and nephrog- enous cord mesoderm containing the presumptive primordial germ cells. may appear in grafts of Hensen's node taken at the head process stage. Willier suggests that results 1 and 2 may be at- tributed to lack of development of a germ- cell transporting mechanism (blood vascular system) in the graft and to lack of some condition in the graft that is essential for proper growth of the gonad. Result 3 may be interpreted on the basis of observations that show the presence of some primordial germ cells in regions posterior to the antero- lateral crescentic zone of the area pellucida that had previously been considered to be their locus. While the experiments are not presented as definitive proof of the extra- gonadal origin of the primordial germ cells in birds, they offer strong support for that view. Evidence that the mature sex cells are derived from the primordial germ cells rather than secondary germ cells is provided by experiments on removal of the left ovary of young chicks. As is well known, the rudimentary right ovary then tends to develop into a testis. Normally the primor- dial germ cells of the rudimentary right gonad disappear after the third week. If removal of the left ovary is performed p
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Keywords: ., bookcentury1900, bookpublisherphiladelphi, booksubjectembryology
