Book - The Pineal Organ (1940) 15
Embryology - 26 Apr 2024    Expand to Translate
|
|---|
| Google Translate - select your language from the list shown below (this will open a new external page) |
|
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt These external translations are automated and may not be accurate. (More? About Translations) |
Gladstone RJ. and Wakeley C. The Pineal Organ. (1940) Bailliere, Tindall & Cox, London. PDF
| Historic Disclaimer - information about historic embryology pages |
|---|
 |
Chapter 15 II. Urochorda
A great deal of interest has been shown in the study of the " single " eye which is found in the ventricular cavity of the cerebral vesicle of the free-swimming larva of A scidia mammillata or Phallusia (Figs. 3, Chap. 1, p. 5, and Fig. 127). Besides the eye, a single statocyst is present in the expanded anterior end of the neural tube, which expansion is usually spoken of as the " sense vesicle." In Phallusia the anterior neuropore remains open for a certain period during the larval stage. Later, regressive changes set in which precede the fixed, plant-like life of the adult animal, and the vesicle with its contained sensory organs disappears, the central nervous system becoming reduced to a small ganglion with associated nerve-fibres. The so-called single eye, according to Froriep, corresponds to the right lateral eye of vertebrates, and he has shown that there is a rudiment of a left eye also present, which is connected by nerve fibres with the neural tube. These fibres join the central nervous system at a point corresponding to the termination in the neural tube of the optic nerve of the right eye. The mode of development of this right lateral eye is indicated in Fig. 127, which shows an enlargement and elongation of certain cells forming a part of the right wall of the cerebral vesicle and a deposit of pigment on their inner or ventricular aspect. This placodal thickening of the wall of the cerebral vesicle is later converted into an optic vesicle, which instead of projecting outwards towards the cutaneous ectoderm becomes enclosed in the ventricular cavity. The " lens " and " retina " are formed by the clear cells which originally lay to the outer side of the pigment-mass and formed part of the wall of the cerebral vesicle. The lens is thus formed from neural ectoderm, not cutaneous ectoderm as in the lateral eyes of vertebrates generally, and there is no invagination of the outer wall of the optic vesicle to form an optic cup. 1 Light can only reach the eye-vesicle through the transparent body-wall and the wall of the cerebral- or sense-vesicle.
1 It may be presumed that the absence of a lens developed from the cutaneous ectoderm is probably due to absence of the invagination of the neural ectoderm to form an optic cup. (R. J. G.)
Owing to the eye of the ascidian larva being single, Salensky and others considered that it could not be homologous with the paired lateral eyes of vertebrates, but was comparable with the parietal eye of vertebrates. Froriep, however, considered that the site of origin of the primary optic plate in the early stages of development of the ascidian larva, namely on the right side of the cerebral vesicle, and the appearance of a similar but more rudimentary optic organ on the left side were presumptive evidence of the organs being homologous with the paired lateral eyes of vertebrates rather than with the " unpaired " median or parietal eye of vertebrate animals. In this connection it is important to note that the statocyst of the ascidian larva is also a single organ, and that in explanation of this it seems likely that the full development of the corresponding area on the opposite side of the head and brain to that on which the existing statocyst has been developed was curtailed or arrested at an early stage. On the other hand, at the time (1906) when Froriep was writing it was not generally appreciated that the " parietal eye " of vertebrates has been derived from paired median organs of which one has been more or less completely suppressed, while the other has assumed a median position {vide Dendy, Gaskell, Hill, A. S. Woodward, Stensio, and others). The demonstration of the bilateral condition of the eye in an ascidian larva is thus not in itself conclusive evidence of the organ not being homologous with the " parietal organ " of vertebrates. Moreover, the structure of the ascidian eye is more like that of the parietal organ of vertebrates than that of the lateral eyes, more especially with regard to the absence of inversion and the formation of the lens from neural ectoderm ; but as we shall see later, these objections are not insuperable when we are dealing with degenerate organs, and the site of origin of the optic plate on the right side of the early ascidian larva is a point in favour of Froriep's contention.
Fig. 127. — Head-end of an Ascidian Embryo, Phallusia mammillata, seen from the Dorsal Aspect. The Epithelium of the Wall of the Cerebral Vesicle on the Right Side is modified so as to form a Retinal or Optic Plate, consisting of Radiating Columnar Cells, the Inner Ends of which are Pigmented.
adh.p. : adhesive papilla.
c. can. : central canal of medullary tube.
cl. v. : " cloacal vesicle " or atrium.
int. : intestine.
n.t. : wall of medullary tube.
ot. : statocyst.
p. : pigment.
r. eye : right eye.
s.v. : interior of sensory vesicle.
Compare with Fig. 3, which represents an older stage of development. (From Korschelt and Heider, after Kowalevsky.)
Fig. 128. — Salpa. A. Lateral view of section. B. Enlarged lateral view of the eye and neighbouring parts (diagrammatic).
at. : wall of atrial cavity. br. : branchia.
c. cr. : ciliated crests on the edge of the branchia. big. : languet.
mo. : mouth.
n. gl. : nerve ganglion.
ces. : oesophagus.
ces. ap. : opening of oesophagus.
ph. : pharynx.
pp. b. : peripharyngeal band.
st. : stomach.
stol. : stolon. cf. : ciliated funnel.
end. : endostyle.
gl. : digestive gland.
gld. : paired " neural gland."
ht. : heart.
int. : intestine.
In B — br. ap. : branchial aperture ; ph. : wall of pharynx.
(After Delage and Herouard, from Parker and Haswell.)
The Eyes of Salpa and Allied Forms of Tunicata
Salpa, Fig. 128, A, belongs to the sub-order Hemimyaria of the order Thaliacea. It has a single horse-shoe shaped eye placed on the superficial aspect of a large oval nerve-ganglion (Fig. 128, B) and just behind the ciliated funnel which is the expanded opening of the ducts of the right and left " neural gland " (n. gl.). The branchial aperture is situated behind the nerve-ganglion. According to von Baer the nerveganglion of tunicates is placed upon the ventral surface of the larva, and does not therefore correspond to the cerebrospinal nervous system of vertebrates. More recently, however, the nervous system of the Prochordata has been studied by Bateson, Harmer, and Hill, and according to Kappers in the Enteropneusts (Ptycodera — Balanoglossus) both invertebrate and vertebrate types of nervous system are present in the same animal ; the invertebrate type being represented by the oesophageal ring and ventromedian neuroepithelium, and the vertebrate type by the medullary tube in the collar and by the frontal and caudal dorsomedial neuroepithelium. Whereas in tunicates the ventral primordium of the nervous system disappears and the dorsal medullary plate closes throughout the whole length of the body, except at the frontal end, where it remains open at the anterior neuropore, which lies close to the ciliated funnel and communicates secondarily with the pharynx.
The median eye of Salpa is sometimes supplemented by accessory eyes. An eye-spot is present in the larva of Botryllus violacea, a colonial type of tunicate ; and in one species of Oikopleura, a minute tunicate which is enclosed in a transparent envelope, like a glasshouse ; in this is a simple light-perceiving organ without pigment which is incorporated with the statocyst, and it may be noted that in Pyrosoma a single statocyst is placed in close relation to the cerebral ganglion and the ciliated funnel. The significance of the disappearance of one member of a pair of senseorgans, such as the otocysts or eye-vesicles in a degenerate type of animal, will become apparent when we consider the reduction in size or disappearance of one member of a pair of median eyes in vertebrates.
| Historic Disclaimer - information about historic embryology pages |
|---|
|
Cite this page: Hill, M.A. (2024, April 26) Embryology Book - The Pineal Organ (1940) 15. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Pineal_Organ_(1940)_15
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
