THE INHIBITOR Y NER VES 7 7 



essentially of a sucking disk with a very powerful muscle, the 

 basilar muscle, and a long piston or tongue with powerful 

 muscles attached to it. The larval form or Ammoccetes shows 

 no sign of any such apparatus, this is formed with great rapidity 

 at the time of transformation." Miss Alcock has shown that 

 the motor nerves to these suctorial muscles, which belong to 

 the trigeminal nerve, are present in the Ammoccetes and supply 

 certain embryonic cells which are found in the tentacles and 

 lower lip. At transformation these cells proliferate amazingly 

 and form muscle fibres, but the nerve fibres in the branches of 

 the trigeminal do not increase in number until they are near 

 the proliferating muscle cells, when they split into an enor- 

 mous number of brush-like filaments in correspondence with 

 the enormous proliferation of the embryonic muscle cells. 

 Each trigeminal motor nerve fibre and the cell in the trigeminal 

 nucleus, from which it arises, becomes very large in size, so that 

 in this trigeminal motor system of Petromyzon we see the same 

 phenomena as is so common in the invertebrate nervous system, 

 namely, large motor nerve cells with large axons which split 

 peripherally into a brush of smaller nerve fibres and so supply a 

 large number of muscle fibres. 



Another most important conclusion results from these in- 

 vestigations of Miss Alcock ; they give no support to the theory 

 of His that in the early embryo the muscle cell is not connected 

 with its motor nerve cell, but that the motor fibre is formed by 

 the growing out of a process from the neuroblast which finally 

 reaches the forming muscle cell, and becomes attached to it. 

 In the Ammoccetes during the larval stage, which constitutes 

 the greater part of the life of the animal, the embryonic motor 

 fibres are there although the muscles are not definable as such, 

 but their cells are in that so-called indifferent embryonic con- 

 dition, when according to His the processes from the neuroblast 

 could not yet have reached them. 



The further evolution of the nerve supply to the voluntary 

 muscles of the vertebrate is I imagine that the splitting process 

 finally involves the nerve cells themselves, and thus forms the 

 comparatively small cells of the anterior horn. 



Another very striking peculiarity of some of these large 

 motor cells of invertebrates, especially manifest in Annelids, 

 has been described by Retzius and others, having been de- 



