ranvier's constrictions in the spinal cord. 97 



clares that no connective tissue surrounds the individual nerve- 

 fibres in the spinal cord, yet it is by no means settled that the 

 medullary sheath of central fibres is not mesoblastic. Blood- 

 vessels enter the cord at a very early date in the life of the 

 embryo/ and it is not certain that the mesoblastic tissue 

 "which forms and surrounds them is limited, as Gierke thinks, 

 to the blood-vessels themselves. Joseph's^ assertion that a con- 

 tinuous network penetrates and binds together axis-cylinder 

 and medullary sheath in the peripheral nerves — a fact of much 

 importance if true — has been denied by Retzius,^ who found 

 the network only in the axis-cylinder. The discovery of 

 Retzius* that Palsemoa squilla has medullated nerves which 

 lack Schwann's sheath and are provided with oval nuclei 

 lying between the medullary sheath and the axis-cylinder has 

 not yet been confirmed. There are finally many who still 

 deny the correctness of the views of His ^ regarding the for- 

 mation of the central nervous system. 



Such reflections are therefore merely suggestive, and are of 

 use only as they emphasize the fact that a solution of many 

 weighty problems is to be found through a sufficient explana- 

 tion of the origin of myelin. 



I take this opportunity to gratefully acknowledge the kind- 

 ness of Professor Flemming, under whose direction this work 

 was done at the Anatomical Institute in Kiel. 



XXV, 1885, p. 533 : — " Alle faserigen Elemente der Stiitzsubstanze Fortsatze 

 voD Gliazellen sind ; andere Faden, als elastiscbe oder Bindegewebsfibrillen, 

 sind durchaus zwischeu den Nervenfaseru niclit zu finden." 



' His found blood-vessels iu the cord between the fourth and fifth week. 

 See " Zur Geschichte des Menschlichen Riickenmarkes und der Nerven- 

 wurzeln," ' Abhandl. der math. phys. Classe der Kgl. Sachs. Gesellsch. der 

 Wissensch.,' 1886, xiii, No. 6. 



^ " Ueber einige Bestandtheile der peripherischen markhaltigen Nerven- 

 faser," 'Sitzuugsber. d. Berlin. Akad.,' Bd. ii, ISSS, p. 1231. 



^ "Der Bau des Axencylinders der Nervenfasern," 'Verhandlung des 

 Biol. Vereins in Stockholm,' Bd. i, Jan., 1889, No. 4. 



■* " Ueber myelinhaltige Nervenfasern bei Evertebraten," ' Verhandl. d. Biol, 

 Ver.,' Stockholm, Bd. i, Dec, 1888, No. 3. 



^ " Every nerve-fibre is a process from its own nerve-cell ; this is its genetic, 

 nutritive, and functional centre." Ilis, 1. c. p. 513. 



VOL. XXXI, PART I. NEW SER. G 



