Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the 3rd German edition by Edward L. Mark.

  • Oscar Hertwig
Date:
1899
Catalogue details

Licence: Public Domain Mark

Credit: Text-book of the embryology of man and mammals / by Oscar Hertwig ; translated from the 3rd German edition by Edward L. Mark. Source: Wellcome Collection.

    661/700 (page 637)
    ward [and ventrad], the anterior limbs more oblic[nely tlian tlie posterior. In both of them the future extensor side lies dorsal, the flexor side ventral. Both the radial and tibial margins witli the thumb and great toe are directed cephalad, the fifth finger and the fifth toe caudad. By this and by the fact that the limbs belong to several trunk- segments are explained certain conditions in the distribution of the nerves of the upper extremity. In the case of the arm “the radial side is supplied with nerves (axillaris, musculo-cutaneus), whose fibres are referable to the fifth, sixth, and seventh cervical nerves. Upon the ulnar side, on the contrary, are found nerves (n. cutaneus medialis, n. medius, and n. ulnaris) whose origin from the lower secondary trunk of the plexus discloses their derivation from the eighth cervical and first dorsal nerves ” (Schwalbe). In the further course of development both limbs alter their original position,—the anterior to a greater extent than the posterior,—in- asmuch as they undergo a torsion around their long axes in opposite directions. In this way the extensor side of the upper arm becomes directed backward [caudad], that of the thigh forward ; radius and thumb are now directed laterad, tibia and great toe mediad. These alterations in position due to torsion are naturally to be taken into account in determining the homologies of the anterior and posterior extremities, so that radius corresponds to tibia and ulna to fibula. In the originally homogeneous cell-mass the fundaments of the skeleton and musculature are gradually differentiated from each other, owing to the fact that the cells acquire a more definite histological character. In this connection the following phenomenon is to be observed :— The parts of the skeleton of the extremity are not all established at the same time, but follow a definite sequence, in somewhat the same manneras, in the development of the axial skeleton, the process of Segmentation begins in front and progresses backward. So in the limbs the proximal skeletal elements (i.e., those which are situated nearer to the trunk) are formed sooner than the distal ones. This is the most strikingly apparent in the case of the fingers and toes. Whereas the first plialanx has been differentiated from the surrounding tissue in embryos of the fifth and sixth week, the second and third are not at that time distinguishable; the ends of the fundaments of fingers and toes still consist of a mass of small cells in process of growth. In this mass the second phalanx is tlien differentiated, and at last the third.