14: SUMMARY OF CURRENT RESEAECHES RELATING TO 



the nerves, then the elements that form ganglion cells, and finally the 

 interstitial enveloping tissue. 



The ganglion cells are secondary formations in the embryonic 

 ganglion-primordia, and their union with the nerve fibres which are 

 already in part formed is a secondary process. The nerve-fibres of the 

 lateral nerves develop independently of any neuroblasts within the 

 syncytial plasma of the primordium, and they enter subsequently into 

 connexion with the ganglion cells. 



Tissue-formation is the visible expression of the organization 

 associated with a function. Nerve-formation is the visible expression 

 of the progressive localization and differentiation of the pre-existing 

 correlation between the sensory stimulus and the contraction. The 

 active cause of the histogenesis is the transmission of the stimulus. 

 There are no preformed nerve-formers, but the formative material of 

 the nerves is determined to its histogenesis and topographical alteration 

 by external influences. The causes of nerve-formation are (1) the 

 active transmission of stimulus which has a histogenetic capacity, and 

 (2) the morphological conditions affecting the course of the nerve. 

 These are some of the main conclusions of an elaborate investigation. 



Eye-muscle Nerves.* — H. V. Neal begins an important study of 

 the morphology of the eye-muscle nerves with an account of the histo- 

 genesis of spinal somatic motor nerves in Squalus embryos. Nerve 

 and muscle are not primarily connected. Previous to the establishment 

 of protoplasmic connexion, the space normally found between somite 

 and neural tube is filled by a vacuolated non-staining, non -protoplasmic 

 liquid containing a relatively small amount of coagulable material. 

 Protoplasmic connexion of somite and tube is established by an 

 amoeboid protoplasmic extrusion from cells in the ventro-lateral wall of 

 the neural tube, forming the " protoplasmic bridges " of Paton or 

 " plasmoderms " of Held. The cell processes which form these con- 

 nexions extend gradually along the median surface of the somite between 

 myotome and sclerotome. "Within these processes the neurofibrils soon 

 make their appearance. This evidence demonstrates the neuroblastic 

 nature of the cells which form the protoplasmic connexions between 

 tube and somite, and their processes are therefore to be regarded 

 as neuraxons. The cell-chain hypothesis of neurogenesis receives no 

 support from the evidence presented in sections of Squalus embryos. 

 The neurofibrillar structure appears in the nerve primordia before any 

 cells are present in them. 



The growth of a nerve-fibre toward its terminal organ does not 

 involve the use and resorption of primary plasmatic paths, but simply 

 the movement and differentiation of the protoplasm of the medullary 

 neuroblast. The most convincing demonstration of the truth of this is 

 afforded by the growth and extension of the processes of Rohon-Beard 

 cells. The numerous cells which, in somewhat advanced stages of histo- 

 genesis, make their appearance in the ventral nerve primordia, are not 

 of mesenchymatous but of medullary origin ; exclusively so in the 



* Journ. Morphol., xxv. (1914) pp. 1-187 (9 pis.). 



