neal: neevous system in squalus acanthias. 219 



iu S. acanthias. Coggi's account differs, however, from that of Dohrn 

 ('90"), who found its relations in Torpedo to be similar to those described 

 by Miss Piatt ('91) for S. acanthias. The relations of the acustico- 

 facialis and glossopharyngeus remain unchanged. For a long time 

 cellular strands persist, showing the primitive relation of these nerves 

 to the constrictions between the encephalomeres IV, V, and VI, re- 

 spectively. 



Some important changes in tbe relations of the neural-crest cells 

 appear in the next (48-somite) stage, and are shown in Plate 3, Figure 

 17. For the first time, we find '• fibrillar" connections of the trigeminus 

 Anlage with the neural tube. Protoplasmic or fibrillar processes ex- 

 tend from the cells which lie opposite the constriction between enceph- 

 alomeres III and IV toward both encephalomeres. It has been stated 

 by some investigators (Miss Piatt, '91, Locy, '95), that this nerve has its 

 origin from the constriction between the neuromeres. Two main roots 

 are differentiated later, an anterior, iu relation with encephalomere III 

 (the "portio minor"), and a posterior, in relation with encephalomere 

 IV (the "portio major"). The nearne-ss of the ganglion cells to the 

 bi'ain wall renders it impossible for me to determine in which direction, 

 whether toward the brain or toward the ganglion, the fibres are first 

 developed. The two chief roots of the trigeminus have been described 

 for other Vertebrates. 



The thalamic and trochlearis portions of the trigeminus Anlage are 

 now much reduced in size, each retaining connection with the rest of 

 the nerve fundament by means of an attenuated protoplasmic fibre. 

 The acustico-facialis Anlage has assumed fibrillar connection with enceph- 

 alomere V, with which it remains connected until the encephalomere 

 disappears. Marshall and Spencer ('81, p. -ISl, '86, p. 100) have stated 

 that in Scyllium "there is an important difference between the fifth and 

 seventh nerves, inasmuch as iu the foi-mer the primaiy root is lost 

 and the secondary alone retained, whilst in the latter both primary and 

 secondary roots are retained up to stage N, and indeed . . . through- 

 out life. The difference between the roots of the fifth and seventh 

 nerves just noticed does not occur in the chick." They also state that 

 in early stages in Scyllium embryos the fifth nerve arises from the brain 

 by three distinct roots, but that in later stages only two roots are found. 

 Their distinction between primary and secondary nerve "roots" is 

 obviously unnecessary, since the only true " roots " are the so called 

 secondary ones. Before these are established we have to do with 

 neural-crest cells, some of which have been shown to be non-nervous 



