By Prof. Dr. Günter H. Rager (auth.), Prof. Dr. Günter H. Rager (eds.)
Wilhelm His, one of many founders of developmental neurobiology, used to be confident "that the techniques of new release and improvement obey basic and straightforward legislation and undergo the overall legislation of nature" (His 1901). hence, we should always manage to locate rapid stipulations, dependencies and principles choosing the de velopment of an natural shape. With this in brain, His (1874) outlined the duty of embryology as follows: "Developmental biology is largely a physiological technological know-how; it has not just to explain how each one person shape develops from the egg, it has to derive this improvement in this sort of manner that every developmental level including all its specialities appears to be like as an important final result of the instantly pre ceding degree . . . provided that developmental biology has given an ideal physiological deriva tion for any given shape, has it the ideal to claim that it has defined this person shape. " the final word goal of a physiological derivation will be that legislation of progress legitimate for natural ,beings should be expressed as mathematical formulae (His 1874). To exemplify this, he formulated a common and in basic terms formal legislation of progress in mathematical phrases making the remark: "I now recommend that the physique shape follows instantly from germinal development and will be derived from the given germinal shape in accordance with the legislation of progress. My curiosity is, for that reason, to start with to realize the legislation of progress empirically and secondly to derive consecutive kinds of the constructing or ganism by means of utilising this law.
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Extra info for Development of the Retinotectal Projection in the Chicken
19). Processes do not extend out in all directions any more; there is a single main dendrite at the apical pole of the cell. It first grows radially and then mainly tangentially producing the stratified appearance of the inner plexiform layer. The other processes of the early multipolar stage disappear. The transition from the multipolar to the mature form of the cell occurs very rapidly. Only one day later (incubation day 9) we see cells located in the central retina which show a single dendrite starting from the apical pole and then spreading tangentially (Fig.
A few thick profiles and growth cones can be seen amongst large bundles of thin axons. These are approximately 5% of the total. 25 Ilm Table 2. Development of myelination Day of ontogenesis 14 15 16 17 18 21 26 28 35 50 78 11 9 Fibres counted No. 7 Table 2 and the histograms shown in Fig. 29 give a quantitative summary of this course of development. Lm). The frequency of each diameter class is indicated on a logarithmic scale. A continuously increasing percentage ofaxons becomes thicker from incubation day 12 onwards when all fibres are small and unmyelinated and growth cones have disappeared.
Representative small areas of the cross-section of the optic nerve of chicken 104 days after hatching taken from the same semi thin section with the same magnification. It is clear that the fibre diameters in the ventro-nasal periphery (Fig. 44) are larger than in the central core (Fig. 42) and in the dorso-temporal periphery (Fig. 43). Bar 25 ,urn 49 o Fig. 45. The cross-section of the optic nerve of a 104-day-old chicken subdivided into standard quadrants. Fibre density (number of fibres/1000 Mm 2) and percentage of unmyelinated fibres are indicated in each quadrant.