The Continuity between the Cavities of the Premandibular Somites and of Rathke's Pocket in Torpedo By SIR GAVIN DE BEER, F.R.S.

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1 279 The Continuity between the Cavities of the Premandibular Somites and of Rathke's Pocket in Torpedo By SIR GAVIN DE BEER, F.R.S. (Correspondant de Vlnstitut de France; Director of the British Museum {Natural History)) SUMMARY The existence of an open communication between the cavity of Rathke's pocket and the cavity of the premandibular somite in Torpedo ocellata at the 7 mm. stage is demonstrated by means of microphotographs. They provide a complete refutation of the claim by B. Wedin that because he was unable to find continuity of open communication between the cavities of Rathke's pocket and of the premandibular somite, therefore E. S. Goodrich was mistaken in homologizing the preoral pit of Amphioxus with the hypophysis of craniates. On the contrary, Goodrich's observations are confirmed and his theory vindicated. IN his paper on the connecting strands between the premandibular cavities and Rathke's pocket, B. Wedin (1951) stated that E. S. Goodrich (1917) and I (de Beer, 1925, 1926) had based his 'theory' that the preoral pit of Amphioxus was homologous with the cavity of the hypophysis or Rathke's pocket of craniates, on A. Dohrn's (1904) descriptions of embryos of Torpedo, and that he (and I) had misunderstood them. In particular, Wedin claimed that Dohrn's expression 'Verbindungen' in his description of the conditions in Torpedo had been mistranslated by Goodrich, and erroneously interpreted to mean that Dohrn had observed continuous hollow connexions between the cavities of the premandibular somites and that of Rathke's pocket. Wedin has re-examined Dohrn's material of Torpedo and found no continuous hollow connexions, but only strands of cells connecting the premandibular cavities with Rathke's pocket in which occasional lumina are present. If Wedin had read Goodrich's and my papers with sufficient attention he would have seen that we based our 'theory' not on Dohrn's descriptions, but on our own positive observations in Torpedo of uninterrupted connexions between the cavities of the premandibular somites and of Rathke's pocket of which we published figures. We found this condition to be variable and the hollowness of the connexion transient. The discovery of the hollow connexion in Torpedo was made by Dr. Elizabeth Fraser in material belonging to J. P. Hill. This material was studied by Goodrich, and Dr. Fraser's observation confirmed and figured (Goodrich, 1917, text-fig. 36, and pi. 28, fig. 15). In my paper (de Beer, 1925, p. 103) I described observations that I had made on material of Torpedo which I had prepared myself, and wrote 'Zuerst stehen die pramandibular Somite und die Hypophyse in offener Verbindung', and gave a figure of a section of an embryo in text-fig. 17, illustrating this fact. In my book (de Beer, 1926) I repeated this in fig. 91, p. 72. Both Goodrich (1917, p. 548) and I (de Beer 1926, p. 35, fig. 31) were able to confirm the existence in Anas of a connexion with an obvious obliterated virtual lumen. [Quarterly Journal of Microscopical Science, Vol. 96, part 2, pp , 1955.]

2 dpsl erp in psr LRprRp FIG. I. A-H, microphotographs of eight consecutive sections through an embryo of Torpedo ocellata, 7 mm. long (series A, slide 1, row 4, sections G. R. de B. D.C.A., Oxford). Photos by M. G. Sawyers am, angle of mouth; cps, cellular strand connecting left and right premandibular somites; csl, cellular strand connecting left premandibular somite to epithelium of Rathke's pocket; csr, cellular strand connecting right premandibular

3 somite to epithelium of Rathke's pocket; dpsl, depression in wall of left premandibular somite; dpsr, depression in wall of right premandibular somite; erp, epithelium of Rathke's pocket; in, posterior wall of infundibulum; Ipp, open left proboscis pore; IRp, left diverticulum of Rathke's pocket; msl, cavity of left mandibular somite; pc, pharyngeal cavity; psl, cavity of left premandibular somite; psr, cavity of right premandibular somite; rrp, right diverticulum of Rathke's pocket; Rp, cavity of Rathke's pocket.

4 282 de Beer The Continuity between the Cavities of the Dohrn's work was referred to by Goodrich together with that of other authors, because they had observed connexions between the premandibular somites and the hypophysis in Torpedo, Raia, Phrynocephalus, Gongylus, and Anas, and these connexions, whether solid or hollow, were obviously homologous with the hollow connexions which we have observed in Torpedo. Wedin has been at some pains to prove that Dohrn's material shows no continuous hollow connexions between the premandibular cavities and the hypophysis, and has reproduced photomicrographs of four embryos of Torpedo, most of which show in the connecting strand of cells a lumen which Wedin asserts does not extend right through. I have no intention of contesting Wedin's descriptions of Dohrn's material. It is difficult to prove a negative proposition, and to invalidate Wedin's argument I only need to reproduce microphotographs of an embryo showing that the hollow connexion between the premandibular cavities and Rathke's pocket is a fact. The embryo is one of Torpedo ocellata, 7 mm. long, cut into horizontal sections, 10 ju. thick (series A, slide 1, row 4, sections in the Department of Zoology and Comparative Anatomy, Oxford). Fig. 1, A, shows a section through the most anterior morphological level of the series. The posterior wall of the infundibulum is seen cut tangentially, in contact ventrally with the thickened epithelium of the anterior portion of Rathke's pocket. On each side of the infundibulum is a premandibular somite with a large cavity. The somite on the left is almost in contact with the epithelium of the left diverticulum of Rathke's pocket; that on the right is a little way distant. Fig. 1, B, which is of the next section, 10 JJL farther back, shows that the left premandibular somite has come into closer contact with the epithelium of Rathke's pocket and that the cavity of the somite shows a conical depression in its wall immediately over the point of contact. Fig. 1, c, shows the open pore between the cavity of the left premandibular somite and the cavity of Rathke's pocket. The limiting membrane of the lining of the cavity of the somite is perfectly continuous with the membrane of the lining of the cavity of Rathke's pocket. The 'proboscis-pore' is open, and its diameter in the transverse plane is approximately 10 /x, which is about the same as its diameter in the sagittal plane, since the open pore is also plainly visible in the next section, shown in Fig. 1, D, at a level 10 fx farther back. In fig. 1, E, the connexion between the left premandibular somite and Rathke's pocket is no longer open, but the wall of the right premandibular somite is seen to approximate to the epithelium of Rathke's pocket on that side. Rathke's pocket itself now appears as a closed cavity since the section is taken dorsally to its communication with the pharyngeal cavity. Fig. 1, F, shows that the right premandibular somite is intimately connected with the epithelium of Rathke's pocket, and cone-shaped depressions can be seen in the walls of the cavity of the somite and of the right diverticulum of Rathke's pocket, immediately at the point of contact. The pore is not open on this side. In the next section, fig. 1, G, the connexion between the right somite and

5 Premandibular Somites and of Rathke's Pocket in Torpedo 283 Rathke's pocket is still intimate, and the cone-shaped depression is also present in the wall of the cavity, but it is no longer seen in the wall of Rathke's pocket. In fig. 1, H, the right somite and Rathke's pocket are still in contact, and the strand can be seen connecting the left and right premandibular somites across the middle line. In the light of these facts I challenge Wedin's statement (1951, p. 75): 'Leider machte Goodrich dabei einen Fehler und unterstellte Dohrn Beobachtungen, welcher dieser nicht gemacht hatte. Dieser Irrtum durfte ausschlaggebende Bedeutung fur die Gedanken Goodrich's haben.' Goodrich made no such mistake, nor that of drawing conclusions from facts which he had not observed himself. His hypothesis that the preoral pit of Amphioxus is homologous with the hypophysis of craniates, and that the open connexions between the premandibular cavities and Rathke's pockets represent 'proboscis-pores', remains unshaken. It might be added that it would still be unshaken if the connexions were solid, for it is a commonplace of comparative anatomy and of embryology that homologous structures may arise or remain solid, or hollow, and so no morphological significance can be ascribed to such differences. In a subsequent paper, Wedin (1952) has described an open connexion between the cavities of the premandibular somite and of Rathke's pocket in Torpedo. REFERENCES DE BEER, G. R., 'Die Geschichte der Pars tuberalis der Pituitardriise.' Anat. Anz., 60, The comparative anatomy, histology, and development of the pituitary body. London (Oliver & Boyd, 1926). DOHRN, A., 'Studien zur Urgeschichte der Wirbeltiere. 24. Die Pramandibularhohle.' Mitt. Zool. Stat. Neapel, 17, 1. GOODRICH, E. S., ' "Proboscis pores" in craniate vertebrates, a suggestion concerning the premandibular somites and hypophysis.' Quart. J. micr. Sci., 62, 539. WEDIN, B., 'Die Verbindungsstrange zwischen den Pramandibularhohlen und der Rathke'schen Tasche.' Proc. Akad. Sci'. Amst., 54, 'Tissue bridges and tubular connections between the premandibular cavities and Rathke's pouch.' Ibid., 55, 416.