Abstract
In making chorio-allantoic transplantations of the entire area pellucida of chick blastoderms in the head-process stage, Willier found that only head structures and heart developed in the grafts and the more posterior trunk structures failed to appear. 1 Since the only differentiation was of parts lying anterior to Hensen's node, the suggestion was made that the node may normally have a rôle in the formation of trunk structures at this stage but for some unknown reason did not realize its capacities in the grafts.
It was then suggested by Dr. B. H. Willier that I make an experimental analysis of the rôle which Hensen's node has in the development of the embryo. The problem was attacked by isolating and transplanting to the chorio-allantoic membrane transverse levels of the area pellucida at different stages of development as follows: (1) the node and other levels of the area pellucida of blastoderms in a stage prior to the appearance of the head-process, (2) the node and other levels of blastoderms in the advanced head-process stage and (3) the node level of embryos ranging from one to 12 somite stages.
The grafts obtained from the transverse levels of the area pellucida of blastoderms, in which a head-process was not yet present, showed that the node level has a great capacity for differentiation and is essentially totipotent. The structures which develop include all levels of the brain, neural cord with ganglia and nerves, epiphysis, eye, notochord, heart, gut and liver, cartilages with attached muscles, integument with feather germs, and mesonephros. Other levels of the area pellucida had very little capacity for differentiation. The transverse level anterior to the node and the streak posterior to the node showed only a development of gut, cartilage, muscle and integument.
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