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EIGIIT FIGt'RESIt is perhaps significant that the central nervous system, presumably the most complex of all organ systems, is the most precocious in its development. It is the purpose of this report to summarize the evidence (from several different types of experimental and observational procedures carried out with chick blastoderms) for the localization and relationship of the neural plate (the primordium of the central nervous system) t o other embryonic structures, particularly to the primitive streak. By means of these proccdures, including especially the marking of epiblast cells with carbon powder, it has been posible to map out more precisely than heretof ore possible the position, shape and average dimensions of the neural plate in primitive streak and head-process blastoderms. Preliminary results have been reported in abstract form (Spratt, '47d). Hamburger arid Hamilton ( '51) stages of development are usually given in parentheses after the author's descriptive name of the stage. Localixatiom at early streak stagesOrdinarily, the neural plate is not clearly visible in living o r fixed and stained blastoderms prior to early head-process stages. However, under conditions of proper lighting and staining the neural plate is visible as a darker, semicircular area around the anterior end of the initial (stage 2) and medium-broad (stage 3 ) streak. Its shape and size approximate that of the plate at the definitive streak stage. 109

Since the time of Dursy (1866) who first extensively investigated the development of the primitive streak in the chick, embryologists have been interested in the manner of origin and growth in length of the streak and its relationship to the developing embryo. The recent experiments of TVillier and his students on blastoderms of the head-process and definitive primitive streak stages have shown that many organforming areas have a definite spatial arrangement about the anterior end of the streak (see Willier and Rawles, '35, and Rawles, '36). Whether a similar relationship may be found at still earlier stages of development of the streak is a problem of considerable importance.The purpose of this study is twofold: (1) to determine the approximate positions of some of the organ-forming regions in blastoderms of pre-and early streak stages, and (2) to esamine the relationship of these regions to the primitive streak as it arises and grows in length. The approach used is that of testing the potency of isolated anterior and posterior pieces of blastoderms to undergo development in vitro upon the surface of a blood plasma clot. The behavior of such isolates at different stages of development has a direct bearing upon I am grateful for the excellent facilities provided a t the University of Rochester where this investigation was begun. G9

The dolitis Hopkiiiv l l n i v f m i t y , Baltiiizore, Mnrylatrd FOURTEEN FIGURESWhen tranverse strips of the pre-and early streak blastoderiri are explanted in vitro, the structures (fore-brain, hindbrain, spinal cord, notochord, etc.) which develop in the isolated pieces correspond to their location along the einbryonic axis at the much later definitive primitive streak stage (Spratt, '42). Thus, most of the neural plate and all of the notochord-forming regions are shown by this method to be located quite far anterior to the original anterior end of the short-beginning primitive streak. These experiments rather strongly indicated then that the anterior end of the short streak, just after its appearance, is only a transitory structure and not the equivalent of the definitive anterior end (Hensen's node). The most plausible manner in which the streak could grow forward and establish those relationships with the notochord and neural plate regions characteristic of the definitive streak stage seemed to be through a progressive differentiation of the epiblast into streak material anterior to the original short streak. In order to obtain more direct evidence on the manner of growth in length of the streak, a series of experiments was begun in which explanted pre-and early streak blastodernis were marked with carbon particles (powdered blood charcoal). The direction, extent, and rate 'It is a pleasure to thank Mr. John Spurheek, artist-photographer, for hie valuable assistance in the preparation of the illustrative material. 359

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