1040 



fibres with a winding course were to be found \n my preparations, 

 althongli I could not find the free end of all of them. Of these 

 figures of growtli at least those that form tiie basis of figs. 22, 25, 

 and 26 may be considered as being absolutely reliable. These figures 

 resemlile, of course, those usually found in preparations of nerves 

 engaged in regeneration (in the regeneration of a peripfieral end 

 of a nerve, being pi-oduced experimentally), Boeke, RaiMon y Cajal, 

 etc. We thus arrive at the interesting fact that in the roots of the 

 segmental nerves of fully intact animals as old as those we are 

 dealing with there really exist neurites engaged in growth, and also 

 a new formation of neurones — a phenomenon that must be con- 

 sidered of fundamental importance for a comprehension of the post- 

 embryonal growth of the whole individual. 



Resume and conclusions. 



r 



The investigations of the material in question have shown that 

 the post-embryonic growth of the peripheral nerves is not due 

 — as far as the axons are concerned — solely to an advancing 

 myelinisation (Donaldson, etc.) and an increase in the thickness of 

 the sej)arate axons, but is also due to an increase in the number 

 of axons. This increase in the number of axons is, however, relatively 

 larger during the earlier than durnig the later post-embryonic period 

 of the animal's development. It is of special interest to note that the 

 results of counting the axons show that the increase in the number 

 of axons goes on for a considerable length of tiuie during the post- 

 embiyonic life of the individual (see the table). This post-embryonic 

 period during which an increase in the number of nerve fibres in 

 the roots of the spinal nerves takes place is many times longer 

 than that during which mitoses can be shown in the spinal ganglia 

 and the spinal cord. 



Investigations carried out with the object of explaining the method 

 in which such a post-embryonal increase in the number of axons 

 arises have shown that it can not be explained by means of 7- or 

 }^-division of the nerve fibres or by assuming that the same nerve 

 cell sends off more than one axon, but that the explanation must 

 be sought in a real increase in the number of the neurones. This 

 increase in the neurones seems to a great extent to be due to the 

 fact that from young cells lying in reserve processes are developed, 

 among which the so-called axons grow out in, among other regions, 

 the roots of the nerves and the peripheral neives. Probably the 

 young cell material in the spinal cord comes from undifferentiated 



