Book - Manual of Human Embryology 16
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XVI. The Development of the Sense-Organs
By F. Keibel.
General Considerations
Not only are stimuli perceived that come to the body from the exterior, but also the internal conditions, such as the position of the joints and the tension of the muscles. For the reception of both kinds of stimuli special apparatus, the sense-organs, may be developed. On account of the surpassing importance and the variety of the external stimuli, the sense-organs for their reception are much more perfectly and variously developed than are those for the perception of internal processes ; indeed it is even doubtful whether the tendon and muscle spindles should be regarded as sense-organs, and the lamellate bodies have recently on good grounds been denied that character (Ramstrom, 1908, and von Schumacher, 1907). Furthermore it is to be noted that the entire external skin possesses in addition to other functions that of a sense-organ ; its development, as well as that of the hairs and hairdisks which belong to it, has been considered in a special chapter.
In the higher sense-organs (the eye and ear) and the olfactory organ the portions which receive the stimuli are derived from the ectoblast. The gustatory organs, as will be shown later, are possibly, indeed probably, derived from the entoblast. As regards the organs of internal sensation, free nerve terminations must be regarded as the stimulus receptors, just as they are in the external skin ; that which at first sight appears to be the sense-organ should really be regarded as accessory apparatus and is developed from the mesoblast.
The Touch=cells, the Lamellate Corpuscles (Vater=Pacinian Corpuscles), the End=bulbs (W. Krause's Corpuscles), the Touch=corpuscles (Meissner's Corpuscles), the Sexual Corpuscles.
It has just been pointed out above that free nerve terminations must be regarded as the stimulus receptors in all the organs mentioned here. As to the development of the accessory apparatus, if the skin and its organs, which have been specially considered, be left out of the question, very little is known. The development of the lamellate corpuscles in man has not, to my knowledge, been investigated in recent times.
Henle and Kolliker made some observations upon them in 1844. These authors recognized them in the sixth month of preg
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DEVELOPMENT OF THE SENSE-ORGANS. 181 nancy as cell masses without any special arrangement of the cells ; in the new-born child they appeared to be quite similar to those of the adult, except that they were smaller and had little or no fluid between the lamella?. W. Krause (1860) found them relatively far developed at a much earlier period. "A corpuscle from the volar surface of the index-finger of a fetus at the end of the fifth month of pregnancy, which he measured, had a length of 0.29 mm. and a breadth of 0.11 mm.; the outermost capsule was quite distinct and the innermost was also recognizable; the rest were merely indicated and possessed an enormous number of oval nuclei arranged lengthwise. Transverse fibres could not be detected. The nuclei just mentioned occurred also in the central cavity, which was 0.225 mm. in length and 0.018 mm. in breadth, , and in its axis was a very distinct, glistening terminal filament which had a diameter of 0.0038 mm. and ended close to the peripheral part of the inner cavity with a slight enlargement." Davydow (1903) has recently investigated the development of the lamellate corpuscles of the cat, but I know of his results only through Weinberg's abstract of them in Schwalbe's Jahresbericht, Neue Folge, Bd. x (1904). According to this the Vater-Pacinian corpuscles in their earliest stages consist of a small number of connective-tissue cells. By rapid increase these cells soon form round or oval cell groups; the roundish elements become altered centrally into elongated ones, and finally all are employed in the formation of the lamella? of the developing Pacinian corpuscle. In the new-born cat a rapid increase in the number of Pacinian corpuscles takes place by budding. In the division the Timofejew apparatus is usually formed from a common fibril-bundle, an arrangement which indicates the possibility of a functional cooperation of several Pacinian corpuscles.
There are also some observations on the development of the touch-corpuscles by W. Krause (1860). He found them in a seven months' fetus in the tips of the papilla? of the vola manus. According to Krause the new-born child possesses in its little fingers and toes just as many touch-corpuscles as does the adult individual, and it must therefore possess a more delicate sense of distance. New touch-corpuscles and especially terminal corpuscles do not form after birth. With these results those of Ranvier (1880-1881) do not quite agree. Eanvier starts with observations made by Langerhans (1873) on young children. According to him the development takes place essentially after birth. In the new-born child one sees in vertical sections through the finger pulp at the summit of most of the papillae, immediately below the first row of epithelial cells, some transverse stria? and, somewhat deeper, an island of roundish mesoblast cells. The transverse stria? represent a nerve telodendron : the nerve ascends directly to the summit of the papilla and there divides into a small number of branches, which terminate in enlargements. These branches lie horizontally, as if they had been forced up through the cell island against the bases of the epithelial cells. In children 50 days old the nerve telodendron has developed greatly, its branches are more numerous and thicker and the mesoblast cells have penetrated between them (Fig. 120). In the sixth month the upper lobe of the composite corpuscle has reached its definitive form. It is well delimited, and in its interior one sees PV^H a certain number of telodendra separated by - S^fof cells that are weakly flattened transversely.
The second lobe is in process of formation. At the base of the first lobe one sees, that is to say, a new nerve telodendron and beneath this is a group of roundish cells that seem about to penetrate it. The contradictions contained in the puscie'ofachiidonodays" observations of Krause and Eanvier are pertreated with gold chloride. } ia p S on i v apparent ones. Eanvier has not n, nerve; b, nerve telodendron, between the branches examined the youngest stages and Krause has of which the cells of the sub- , • ^ \ n n i i*»ji n jacent node are penetrat- not considered the development of the finer S-^ioJ^^E Parts of the small organ. Krause (1869) has ri I is- a 7 ti ) aild Wjss ' Leip " a ^ S0 i nves tigated the development of the spherical end-bulbs in the conjunctiva bulbi of man; in a six months' fetus they had the appearance of masses of nuclei or cells, nevertheless they already possessed a distinct investing membrane.
The Epibranchial Sense Organs
Epithelial thickenings that may be found dorsal to the branchial clefts of embryos of from 4 to 12 mm. may be regarded as rudimentary sense-organs. These thickenings, which are termed sense-placodes, occur in connection with the vagus, glossopharyngeal, and facial nerves, and less distinctly with the trigeminus, cells being given off from the ganglia of these nerves: they may then be transformed into small epithelial pouches. See regarding them Keibel and Elze, Normentafel zur Entwicklungsgeschichte des Menschen (1908), Plates 10-45, Ingalls (1907), and the chapter in this work on the development of the peripheral nervous system. The placodes have probably a great importance from the standpoint of comparative anatomy and embryology. It is supposed that the auditory and olfactory organs have been formed from such placodes, and the lens of the eye has also been derived from a placode; this may have been originally the actual sense-organ. On this point see Brachet (1907a and 1907b).
