Differentiation versus cleavage in chorio-allantoic grafts.

A considerable amount of data obtained in the course of a large series of experiments upon the embryo of the fowl has lead to the conclusion that there is a great amount of independence between the processes involved in cell division and those concerned with differentiation in ontogeny. The conclusions are based upon the fact that when parts of the embryo are grafted at early stages, the size of the resulting organ is much smaller than that of organ segregates grafted at later stages. There is a direct relationship expressed in the series studied. Mention is made of the development of a number of organs in grafts and controls. The eye is selected for detailed discussion because of the fact that the differentiation of the subordinate parts is clear-cut, and models may easily be made of such organs for subsequent weighing and comparison. While there is a marked discrepancy between the sizes of the organs arising in grafts made at different developmental stages, the differentiation of the subordinate parts is essentially similar to that in the controls. The behavior in the chick is, therefore, similar to that already observed in some of the invertebrate groups. Such a method of attack may yield valuable information concerning the intimate nature of the processes involved in each event, viz. cell division, and cell differentiation, which may, very conceivably aid in their interpretation of these events.