Ectopic Differentiation of Ears in Secondary Embryos of Fundulus.

Introduction.-Although it was first demonstrated almost twenty years ago1 that fragments of the early embryonic shield of Fundulus differentiate successfully after implantation to the extra-embryonic membrane of hosts the same age as the donors, no systematic analysis has subsequently been made of the comparative capacities for differentiation of various -portions of the shield after such transplantation. Accordingly, an investigation is currently in progress for the purpose of studying on the extra-embryonic epithelium the development of fragments representing four different antero-posterior levels of the Fundulus shields. While final description and interpretation of the results must await completion of the histological study of the experimental embryos, inspection in the living material of the patterns of differentiation accomplished and evoked by the grafts reveals some features of sufficient significance to warrant their presentation at this time. Experimental Procedure.-The grafts consisted of fragments obtained by dividing stage 13 or stage 142 shields antero-posteriorly into four parts, of approximately equal length (Fig. 1). These were implanted on the extra-embryonic membrane of hosts approximately the same age. All layers constituting the shield were included in the grafts. The hosts were raised to stage 31 or stage 32, the prehatching or hatching stages, respectively. Observations.-The secondary structures identifiable in the living condition which were differentiated by the grafts or in association with them included brain, spinal cord, eye and lens, ear, heart, pericardium, notochord, segmented musculature, urinary bladder, peritoneum, and pectoral and longitudinal fins. When similar grafts, in another experimental series,3 had been implanted into the shield rather than on extra-embryonic membrane heart, pericardium, bladder, peritoneum and longitudinal fin were not differentiated, and pectoral fin was formed with far less frequency. Furthermore, as far as the components of the secondary aggregates which can be identified in the live material are concerned, the variety of structures differentiated as a result of implantation of grafts derived from any specific level seemed considerably wider after transplantation to extra-embryonic than to embryonic regions; this was particularly evident for grafts from the posterior parts of the shield. Of equal interest with the specific content of the secondary masses is the arrangement of their constituent organs within them. In a number of cases, following the extra-embryonic implantation of the fragments designated 3 and 4 in Figure 1, the secondary structures were organized to form entities resembling trunk and tail, in which the principal axial structures exhibited their typical topographic relationships to one another. These represented secondary embryos which, though smaller than their primary counterparts, were sometimes of remarkable quality (Figs. 2 and 3).