Volume 2

Text-book of embryology / edited by Walter Heape.

Date:
1914-
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Text-book of embryology / edited by Walter Heape. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

    95/616 (page 79)
    The flask-glands of Lung-fishes and Amphibians develop in the lirst instance as solid local proliferations of the deep layers of the epidermis which grow down into the subjacent connective tissue of t lie dermis and form a lumen by secondary excavation. The fully developed ilask-gland is ensheathed in a coat of smooth muscle-fibres and it is an interesting fact that these are belieVed to be developed from the ectodermal cells of the gland-rudiment. Cement-organs of apparently ectodermal origin occur in two out of the three surviving lung-fishes—Frotoptenos and Lepidosiren— and form conspicuous features during late embryonic and larval life (see Fig. 200, F, Chap. VII.). In an embryo of Lepidosiren three days before hatching the cement-organ forms a crescentic structure stretching across the mid- FiG. 48.—Embryos ami larvae of Bufo vulgaris to sliow the cemeiit-orgau upon the ventral surface. (After Thiele, 1887.) ventral hne with its concavity forwards, just behind the position in which the mouth will appear later. About stages 32-34 the organ is at its maximum development and forms a large prominently pro- jecting structure ventrally below the opercular region. Towards the end of larval life the cement-organ commences to atrophy, the pi'ocess being furthered by its invasion by crowds of phagocytes, and in a snort time the organ has completely disappeared. The cement-organ is a derivative of the deep layer of the epi- dermis. It commences as a slight thickening of this layer (Fig. 47, A) the cells assuming a tall columnar form. These columnar cells become the secretory cells while the superficial layer of the ectoderm breaks down over them so as to expose their outer ends (^ig. 4/, B and 0). It is to be noted that there is no trace whatever ot any connexion of this cement-organ with the endoderm: it is ontogenetically a purely ectodermal structure (see, however, p. 181). Cement-organs of very similar appearance are found in tlie larvae