. Text-book of embryology. Embryology. II NERVE DEVELOPMENT 111 above description—the existence of the motor trunk in the form of a bridge of protoplasm between myotome and spinal cord at a time when they are still in close proximity—has been confirmed for an- other very primitive group of Gnathostomes, the Elasmobranchs (Paton, 1907), as is shown in Fig. 62. It will now be convenient to review the facts just described for Lepidosiren in relation to the general theory of nerve-development. (1) It is clear in the first place that the His view is put out of court, seeing that before there is
. Text-book of embryology. Embryology. II NERVE DEVELOPMENT 111 above description—the existence of the motor trunk in the form of a bridge of protoplasm between myotome and spinal cord at a time when they are still in close proximity—has been confirmed for an- other very primitive group of Gnathostomes, the Elasmobranchs (Paton, 1907), as is shown in Fig. 62. It will now be convenient to review the facts just described for Lepidosiren in relation to the general theory of nerve-development. (1) It is clear in the first place that the His view is put out of court, seeing that before there is any development of nerve-fibrils the motor nerve-trunk already exists in the form of a bridge of proto- plasm connecting spinal cord and myotome. (2) It is equally clear that the Balfour view is inapplicable: the nerve-rudiment cannot in early stages by any possibility be re- garded as a chain of cells, seeing that its total length is greatly less than the diameter of a single cell-nucleus. (3) While the nerve-rudiment forms a primary connexion be- tween spinal cord and myotome, in the sense that it is in existence before these organs begin to recede from one another, there is no evidence by which the connexion can be traced back to intercellular bridges or plasmodesms (Stras- burger, 1901) of early, segmen- tation, stages in the development of the egg, as would be the case according to Hensen's theory. (4) The primitive protoplasmic bridge gradually becomes fibril- lated but there is no means of determining with any degree of certainty how these fibrils are developed. It is suggested1 that the development of the actual nerve-fibril is simply the gradual coming into view of a pathway produced by the repeated passage of nerve impulses over a given route. It is clear from the study of the simpler organisms that one of the most ancient properties of living protoplasm is that of the trans- mission of impulses through its substance. Although nothing is really known as to t
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