. Text-book of embryology. Embryology. 106 EMBEYOLOGY OF THE LOWER VERTEBRATES ch. incomplete separation of the cells during the processes of segmenta- tion and cell division—the view in his hands came to approach the next view to be mentioned that of Hensen. (3) The Hensen View (Primitive continuity theory).—This view has found its strength in general physiological considerations rather than in convincing facts of observation. According to Hensen (1864,1868,1903) the nerve which connects centre with end-organ is a primary connexion which has been there from the beginning. It existed first as
. Text-book of embryology. Embryology. 106 EMBEYOLOGY OF THE LOWER VERTEBRATES ch. incomplete separation of the cells during the processes of segmenta- tion and cell division—the view in his hands came to approach the next view to be mentioned that of Hensen. (3) The Hensen View (Primitive continuity theory).—This view has found its strength in general physiological considerations rather than in convincing facts of observation. According to Hensen (1864,1868,1903) the nerve which connects centre with end-organ is a primary connexion which has been there from the beginning. It existed first as a simple bridge of protoplasm, such bridges being present between the various cells of the body owing to the fact that the processes of segmentation and cell division are not complete so as to lead to absolute isolation of the cells or segments from one another. According to this view the growth in length of a nerve-trunk is simply the extension of a pre - existing bridge, as the organs at its two extremities — centre and end- organ — are pushed apart from ' one an- other^ during the course of development. Fig. 59.—Illustrating Hensen's view of the origin of peri- Hensen figures in pheral nerves. The section is taken from a 9-day Rabbit ],jo -nfmers (Tiff 59^1 embryo, and passes through the trunk region. (After , , , . , , n Hensen, 1903.) what he takes to be such nerve-rudiments, in the form of numerous fine filamentous structures passing across the space between spinal cord and myotome. There is however no evidence to show that these filaments have anything to do with nerve-trunks. Although for this reason it is impossible to accept the main observational basis of Hensen's view, that does not neces- sarily invalidate the physiological considerations which may be held to give an a priori probability to the correctness of his general theory. The three views which have been outlined above were fashioned by their respective authors long ago as embryological science goes
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