Chorio-Allantoic Grafts of Entire and Fragmented Blastoderms of the Chick

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Abstract
1. Chorio-allantoic grafts were made of entire unincubated blastoderms of the chick. The differentiation which occurred in the grafted blastoderms involved the formation of the following organs and tissues: central nervous tissue, cartilage, mesonephros, smooth muscle of the gut wall, gut, liver, probably epidermis. Cardiac muscle is probably present. The notochord is probably absent. There was practically no resemblance to normal anatomy. 2. The central nervous tissue was always found in the form of tubes. It was differentiated into grey and white matter, the former nearer the lumen of the tubes, the latter more peripheral. A germinal layer lines the tubes. Nerve trunks run off from the nerve tubes, but these show no approach to the normal anatomical arrangement. There has been differential determination between brain and spinal cord; this is indicated by the presence of an eye in one specimen. 3. The endoderm is present as a series of tubes, probably histologically differentiated into intra- and extra-embryonic endoderm. The presence of the liver indicates differential determination within the alimentary canal. 4. Similar grafts were made of fragments of unincubated blastoderms. In connection with the interpretation of these, two important sources of error are indicated: (a) the doubt which always exists in regard to the orientation of the cut which divides the blastoderm, (b) the variability in the stage attained by chick embryos of equal age. The grafts were thirds of blastoderm, with the exception of one which was a quarter. The plane of section was always parallel to the long axis of the egg, i.e. supposedly transverse to the long axis of the embryo. 5. The following organs and tissues differentiated in the grafts of blastoderm fragments: central nervous system with tubular arrangement, differentiated into grey and white matter, and giving off a few nerve trunks, endoderm in the form of tubes apparently differentiated into extra- and intra-embryonic endoderm, and also apparently with differential determination within the alimentary tract as indicated by the presence of liver in one case. Cardiac muscle is very doubtfully identified in several cases. It should be noted that in several cases the only apparent differentiation was of endoderm tubes. 6. It is concluded that: (a) The inhibition of differentiation seen in grafts of fragments of blastoderms is a consequence of the fragmentation and not of the grafting per Se, i.e. it is a result of the isolation of the graft from the embryo as a whole. (b) It is probable that the extent of this inhibition varies inversely with the size of the fragment. (c) Since a high grade of histological differentiation may be attained in the absence of anything even remotely suggestive of a normal gross anatomy, it is evident that it is not dependent thereon.