File:Image from page 210 of "Bulletin" (1971).jpg

English: Title: Bulletin

Identifier: bulletin8628sout Year: 1971 (1970s) Authors: Southern California Academy of Sciences Subjects: Science Publisher: Los Angeles, Calif. : The Academy Contributing Library: California Academy of Sciences Library Digitizing Sponsor: California Academy of Sciences Library

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Text Appearing Before Image: CAUDAL SPINE REPLACEMENT IN ROUND STINGRAYS 121 â » Ec band

Text Appearing After Image: Fig. 3. Photomicrographs of primordial spine bud in the round stingray, a) A cross section of the dorsal surface of the tail of Urobatis halleri showing a developing spine primordium. The inserted boxes (b-d) indicate areas that are shown in figures 3b-d at higher magnification. Outer layer of ectoderm (Ec); endoderm (En); cuboidal ectoderm (Cu); ectodermally-derived pseudostratified colum- nar epithelium (Pe); mesodermally-derived pseudostratified columnar epithelium (Pm); mesenchymal- like cells (Mc); enameloid matrix (Em); and the advancing edge of the developing enamel organ (Ae). Histology The newly formed dorsal integument that gave rise to the spine bud in Urobatis halleri appeared as a gelatinous, opaque grayish-purple tissue. Histological anal- ysis of this region indicated that spine formation began with the formation of a central, longitudinal depression in the ectoderm (Fig. 3a,b). A one to two cell thick band of basal layer cells, supported by an aggregation of mesenchymal cells, extended from this depression ventrally into the dermis (Fig. 3b). Primordial spine formation began a short distance into the dermis where this band bifurcated to form a sheet of more or less cuboidal cells. The advancement of this sheet's leading edges was at first primarily ventro-lateral, but then growth turned inward so as to enclose a region of the mesenchyme. As this sheet of cells progressed through the dermis, its advancing edges recurved upon themselves and gave rise to an interior adjacent layer of pseudostratified columnar epithelium (Fig. 3c). The cells of the columnar layer were characterized by the presence of large numbers of azurophilic secretory granules. These granules were predominantly located be-

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