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Because of the large size of its cell bodies, the abdominal (visceral) ganglion of the sea hare, Aplysia, has proven a useful preparation for biophysical and pharmacological studies of central neurons (1, 2, 7, 18). However, the systematic study of the organizational properties of this ganglion has been seriously limited by an inability to recognize more than a very few specific cells from one preparation to another (3, 9, 17, 19). In order to overcome this difficulty we have identified twenty-nine of the largest cells in the ganglion. The criteria for identification include: location of the cell in the ganglion, peripheral distribution of its efferent axon, its synaptic input, spontaneous activity, synaptic connections with other identifiable cells, and its appearance under the light and electron microscopes. The usual position of each of the 29 identified cells is indicated in Fig. 1, which is an idealized drawing of the dorsal and ventral surfaces of the ganglion. The availability of these identifiable cells, which can now be studied with both physiological and morphological methods, makes it possible to correlate cellular structure and function in this ganglion. The present report describes a combined electrophysiological and morphological study on a cluster of distinctive identified cells. A more detailed report on all the identifiable cells is in progress (8). Under the dissecting microscope all neurons on the left side of the ganglion, identified as well as unidentified, appear translucent and orangecolored. On the right side, in addition to the colored neurons, there are a number of cells which are opaque and white. The white cells are stippled in Fig. 1 and are designated R3 to R15. Eleven of these ceils, R3 to R13, form a cluster in the rostral part of the ganglion. The number of clearly visible white cells in this cluster may vary, in different ganglia, from six to eleven. Two solitary white cells, R14 and R15, are found in the caudal part of the ganglia. Because they differ in certain respects from the white cells in the rostral cluster, the two caudal white cells will be described in more detail in the later paper (8). In Aplvsia californica, the white cells contain droplets that are stained faintly by chromalum hematoxylin or paraldehyde fuchsin, two basic stains for the demonstration of neurosecretory material in the light microscope. In 1935 and 1937, Berta Scharrer reported that some cells in the abdominal ganglion of either Aplysia limacina or Aplysia depilans contained many droplets that were presumed to indicate neurosecretion (13, 14). I t is likely that the white cells in A. californica are homologous to the neurosecretory ceils described by Scharrer in the European species of Aplysia. In osmium tetroxide-fixed and plastic embedded material, the white cells can be located by their large size and distinctive position. I t is possible to further characterize them by the presence of cytoplasmic granules. The granules are round, vary in diameter from 1000 to 4000 A, and possess large osmiophilic cores (Fig. 2). Thus, they resemble the "elementary neurosecretory granules" seen in unidentified cells in this ganglion by Rosenbluth (12) and Simpson et al. (16), or in the cells of established neuroendocrine systems such as the supraoptico-hypophysial system in vertebrates (11). I t is presumed that these granules, either

A MORPHOLOGICAL AND FUNCTIONAL STUDY ON A CLUSTER OF IDENTIFIABLE NEUROSECRETORY CELLS IN THE ABDOMINAL GANGLION OF APLYSIA CALIFORNICA