Book - The Pineal Organ (1940) 16
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Gladstone RJ. and Wakeley C. The Pineal Organ. (1940) Bailliere, Tindall & Cox, London. PDF
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Chapter 16 Cephalochorda — Acraniata
The pigment cells and light-percipient organs found in the lancelet fish, or Amphioxus, have been specially studied by Hesse and Joseph ; the latter has shown that the pigment spot beneath the unpaired olfactory lobe, which has been described as the eye spot or median eye of the lancelet, consists merely of a cluster of cylindrical cells, filled with pigment granules which occupy the extreme anterior end of the " so-called " cerebral ventricle, and that in his opinion there is no structural resemblance between this group of pigment cells and even the simplest type of vertebrate eye. He also described similar cells in the dorsal region of the anterior part of the spinal cord. Neither the cells of the pigment spot or of the larger group at the anterior end of the spinal cord (cells of Joseph) are in relation with neuro-sensory cells, and it has been proved experimentally that neither are sensitive to light. Moreover, if these regions are removed by decapitation, the body of the animal still reacts to light. The explanation of this lies in the existence of the organs of Hesse (Fig. 129, A and B, p. 179), which consist of a series of single large neuro-sensory cells capped by a crescentic pigment layer which covers over one-half of the organ ; a nerve process arises from the sensory cell, either from beneath the edge of the pigment cap, or opposite the centre of the cap. The pigment is arranged in three distinct layers, a thick band in the middle and two thin layers external and internal situated respectively superficial and deep to the middle layer. Between the pigment layers are two unpigmented strata which are believed by Boeke to contain neurofibrillar in the cell-protoplasm.
The arrangement of the organs of Hesse in the spinal cord is remarkable. They are situated ventral and lateral to the central canal of the spinal cord commencing at the third or fourth segment, where they are most numerous and becoming less frequent towards the tail end. It is said also that on the left side of the cord the " eyes " are directed upward or dorsalward ; on the right side, downward or ventralward (Fig. 129, A). Kappers remarks that as the animal usually lies on one side it would in this position perceive light reaching it in a horizontal plane. Beside the organs of Hesse, other neuro-sensory cells are found which lie in the ependyma lining the central canal, the neurites of which pass into the spinal cord (Edinger). This position of neuro-sensory cells is explained by the primary origin of the spinal cord from an open medullary plate lying on the superficial surface of the embryo, in the same morphological situation as the neuro-genetic or neuro-sensory epithelium of invertebrates. Similar cells have also been found in Ptychodera (a species allied to Balanoglossus) by van der Horst, in the collar region and extending forward and backward from this situation into the dorsal region.
Fig. 129. — A : Lateral View of the Head-end of Amphioxus, showing the Anterior Pigment Spot, the Dorsal Pigment Area of the Spinal Cord and the Organs of Hesse. B : Section through one of the Organs of Hesse.
d.p.a. : dorsal pigment area of spinal cord. p. :
H. org. : organs of Hesse. p.s.
n. : nerve. r. : N. ch. : notochord.
pigment.
anterior pigment spot (eye). rods.
With regard to Joseph's statement concerning the histological structure of the pigment-spot of Amphioxas, it must be remembered that the cephalic region of this animal, which burrows head-downward in the sand, is degenerate, and that notwithstanding the want of resemblance of the " spot " to either a lateral or median vertebrate eye, the presence of pigment in this situation is significant,' as pigment is one of the most persistent of the tissue-elements both from the ontogenetic and phylogenetic standpoints. It is quite common in aborted and macerated human embryos or foetuses to find the eyes represented merely by a mass of degenerate pigment cells ; and Klinckowstroem has shown that the position of the pineal organ in sea-gulls and other birds is represented by a mass of irregularly distributed pigment cells in the parietal region (Chap. 3, P- 75)
1 See p. 215.
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Cite this page: Hill, M.A. (2024, April 24) Embryology Book - The Pineal Organ (1940) 16. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Pineal_Organ_(1940)_16
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
