Experimental investigation of the endoderm in early stages of development shows that the endodermal organ rudiments, like those derived from other germinal layers, are not initially determined, but that the endodermal cells destined to participate in the formation of the various organ rudiments are no more determined for their respective fates than are the cells of the other germinal layers.
In the earlier stages of development, their fate is a function of the position that each cell or group of cells occupies in the embryo as a whole. This can be proved by isolating parts of the presumptive endoderm and cultivating them apart from the rest of the embryo or by transplanting them into an abnormal position.
In an extensive series of experiments, pieces of presumptive endoderm of young gastrulae were cultivated in the “Holtfreter solution.” Various tissues were observed to differentiate from such isolated pieces; some conformed to the normal destiny of the isolated parts, and some did not.
The range of differentiations included not only orobranchial epithelium, stomach epithelium, liver, pancreas, and intestine, but also notochord and muscle, which should not have developed from the presumptive endoderm if it had kept its prospective significance.
ADVERTISEMENTS: