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A NOTE ON ENDOTHELIUM. 391 cavity and mouth, but the gastrula-stomach and orifice of invagination ; the latter clos.es up, and the pedicle so formed becomes the rectum, which terminates blindly. E. With a good English quarter, or, better, with Hartnack's 10 immersion, and the use of osmic acid, the cilia which cause the rotation may be seen. They are disposed on an annular band, the commencing velum. F. The annular ridge has nothing to do with the mantle's margin, but is the velum. G. The new in-sinking has no connection with the anus ov rectum, which latter already is taking shape in the pedicle of invagination. It is the " shell-gland," a structure com- mon to many embryo mollusca, but hitherto unrecognised. H. The so-called large yelk-spheres are not now first formed, but have been there all the time, forming the wall of the invaginated gastrula-stomach. They now undergo im- portant segregative changes, and present the appearance of large clear globules, covered in by a fine granular reticulum. i. The alimentary canal is from the first bent, the csecal termination of the rectum lying a little forward, and not opposite the mouth. j . The shell, as an exceedingly delicate membrane on the surface of the shell-patch (in the centre of which lies the shell-gland), is observable long before this. K. The mouth never pushes forward, but rather becomes sunk and enclosed by the increasing development of the border of the velum, where it overhangs the mouth. This part of the velum forms the subtentacular lobes of the adult. L. It can be seen at a very much earlier period. A NOTE on ENDOTHELIUM. By JOHN CAVAFY, M.D., Assistant-Physician and Lecturer on Physiology at St. George's Hospital. IN the last number of this Journal 1 there is a paper by Dr. M. Foster, " On the term Endothelium," in which he gives various reasons against the further use of this word in histological terminology. He objects to the word, both because its etymology is " of the most grotesque kind," and also on far more important anatomical grounds. Much that is brought forward by Dr. Foster is, doubtless, true; but some portion of his statement must, I think, he ' 'Quart, Jouru. ofMic, Sci.,' 1S74, p, 219, 392 DR. CAVAFY. admitted to be mistaken, and he has completely passed over some important reasons which exist for maintaining the separate terms originally proposed by His. In the first place, the word is not quite so ridiculous, ety- mologically, as it is represented to be by Dr. Foster. The word endotheUum is obviously a contraction of endo-epithe- lium, and means an epithelium which is within (IvSov or euros, inttts) or internal, i. e. it means precisely what His intended to signify, and is only so far a misnomer as (epi- thelium is concerned, since it covers "• surfaces of which, one great characteristic is that they are devoid of papillee j " but, as Dr. Poster says, this extension of the meaning of Ruysch's original terms epitheUs (of which epitheUda is the accusative case) and epithelia " may be easily allowed." Endothelium does not mean " that which is inside a papilla" any more than entoderm means ?f that which is inside a skin j " endo- plast, " that which is inside a formation;" or even entozoon, "that which is inside an animal." The real meaning of these terms Dr. Foster will, doubtless, admit to bs " a n internal skin, formation (nucleus), and animal." The etymological objections, therefore, do not hold. Let us now consider, shortly, whether the physiological differences between epithelium and endothelium are sufficient to warrant the use of separate terms. Dr. Foster says that endothelium cannot be employed to denote the epithelium derived from the mesoblast, " for it would then include structures still called epithelium, aod differing in no essential characters from the epithelium derived directly from the hypoblast." But we should not forget that the mesoblastic origin of genito-urinary epithe- lium may be more apparent than real. Waldeyer1 has sug- gested that its real origin is most probably from cells of the epiblast, which have become mixed with those of the meso- blast at the time of the formation of the primitive groove. This view is not, of course, susceptible of actual demonstra- tion ; but it is known, at any rate, that the mesoblast becomes closely fused with the epiblast in the region of the axial cord of His, and then again separates; and that it is after this separation that the Woiffian and Miillerian ducts are formed. It is therefore quite possible that the cells lining these organs and their derivatives may be really derived from the epiblast. Physiologically, at any rate, they are true epithe- lium, and not endothelium, i- e. they are concerned with secretion, which is never the case with endothelium. The email quantity of fluid which bathes the surface of serous 1 'Eierstook und Ei,' pp. 113—114. NOTE ON ENDOTHELIUM. 393 and synovial membranes is a filtration or transudation, not a true secretion in the ordinary physiological sense of the word, for endothelium never forms glands, whereas a share in gland- formation, and, consequently, secretion, is a constant charac- teristic of epithelium, whether derived from epiblast or hypoblast. As Dr. Foster says, continuity " affords no argument whatever for classing . . . . together. "We find continuity everywhere." But in the case of endothelium we find in one most important instance, not only continuity, but something more. What takes place in the growth and development of capillary blood-vessels and lymphatics ? We have in each case a vacuolation and excavation of connective-tissue-cor- puscles, whose protoplasm becomes ultimately changed into endothelial plates, and we may find every transitional form between a flattened connective-tissue-corpuscle occupying the lymph canalicular system and the endothelium forming the wall of the lymphatic vessel with which it communicates. • We have here, therefore, to do with convertibility as well as continuity; and if Dr. Foster admits that the connective tissues form a natural group derived from the mesoblast, of which convertibility of one member into another (well seen in pathological conditions) is one of the most striking cha- racteristics, I do not see how he can exclude endothelium from a very close connection and relationship with the members of that group. In fact, the most recent researches on connective tissues have established a nearly complete identity between endothelium and connective-tissue»cor- puscles, and this identity is strikingly evidenced by the close similarity of the pathological changes which they both undergo.1 Now, in the case of epithelium, not only has no such connection with connective tissues been observed, but there are marked differences between their respective pathological changes. The views of Heidenhain as to the continuity of the cylindrical epithelium covering the villa of the small intestine with branched connective-tissue-cor- puscles in the stroma have been rendered more than 1 " Les cellules du tissu conjonctif sonb plates, contienuejit des noyaux gats, et completement impossible de sur les faiaceaux cellule endotlieiiale serait sont simplement appliquSes recounaitre une du tis3U conjonctif. isolee de la plupart des cellule* du tissu conjonctif."—Ranvier, art. "Epithelium," 'Nouveau Dictionnaire de M^deeine,' vol. xiii, p, 687. "Les alterations des s6reuses . . . prSsentent une analogie com- plete avec celles du tiseu coajonctif."—Cornil and Ranvier, ' Manuel d'Histologie pathologique,' p. 456. 394, E. A. SCHAFER. doubtful by the important observations of Mr. Watney 1 on intestinal absorption, which he has shown to take place, not through the epithelium itself, but by pseudo-stomata formed by processes of connective-tissue-corpuscles, which project between the epithelium, with which they are never anato- mically continuous. We have, then, in the case of endothelium, not only an undoubted mesoblastic origin, but a close relationship with connective tissues; its cells, moreover, never form glands nor secrete. Epithelium, on the other hand, has a doubtful derivation from the mesoblast; gland-formation and secretion are among its most constant and striking characteristics, and it has absolutely no relationship with the connective tissues. Notwithstanding, therefore, Dr. Foster's statement (p. 292) that, " In short, there is no reason why the cells spoken of as forming endothelium should have a common title, distinct from the general term epithelium," I must confess that I am not convinced, but think that the reasons I have given above are sufficient to render the use of sepa- rate ' terms at least opportune—" im Interesse physiolo- gischen Verstandnisses." DESCRIPTION of an APPARATUS2 for MAINTAINING a CON- STANT TEMPERATURE under the MICROSCOPE. By E. A. SCHAFER, Assistant-Professor of Physiology in University College, London. THE necessity of having the means of conveniently, but at the same time accurately, maintaining objects, especially the living tissues, under observation at a uniform temperature (generally that of the body) becomes more obvious every day. The existing methods of effecting this are, as a rule, not sufficiently accurate for exact investigations; and, on the other hand, the more accurate modes are frequently in- convenient of application. For example, the apparatus described by Strieker and Burdon-Sanderson in this Journal for 1870,—although it is possible by its aid to maintain a constant temperature under the microscope for a con- siderable time,—yet requires that there should be a vessel of water constantly boiling near the observer, and that the water in this vessel should be maintained at a uni- form level, necessitating a supply tube from a cistern, and 1 ' Proc. Roy. Soc.,' vol. xxii, p. 293. 8 Made for me by Mr, Casella, of 147, Holborn Bars.
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