476 



Bashford Dean Memorial Volume 



Brohmer (1909) described briefly the cranial nerves of a 25'min. embryo. The account of 

 the cranial nerves by AUis (1923) is, as the author states, incomplete. 



The general plan of the cranial nerves of vertebrates is best revealed in their em' 

 bryos. For the embryo of Scyllium, this plan is set forth diagrammatically in Text'figure 

 117. A somewhat comparable figure for Chlamydoselachus, based on a single embryo, 

 is supplied by Text'figure 118, after Brohmer. 



TUC 



hXTh 



Text'figure 117. 



Diagram of the segmentation of the head in an embryo of Sc-yWium canicula. The myotomes are 



longitudinally striated, the nerves black, and the scleromeres dotted. The cartilaginous visceral 



arches, also the optic capsule and the nasal sac, are represented by dotted outlines. 



I' VI, gill-slits; I-ll, somites, prootic from 3 forwards, and metaotic from 4 backwards; a, auditory nerve; ah, abducens nerve; 

 ac, auditory capsule ; ah, anterior head cavity ; c, coelom in lateral plate mesoblast ; cr, limit of cranial region ; /, facial nerve ; 

 gl, glossopharyngeal nerve; ha, hyoid cartilaginous arch; hm, hypoglossal muscles from myotomes of somites 6, 7, 8; h)i, 

 hypoglossal complex nerve; !a, lamina antotica; m, mouth; ml, second metaotic myotome; m6, sixth metaotic myotome; 

 ma, mandibular cartilaginous arch; mb, muscle bud to pectoral fin; nc, nasal capsule, continuous with trabecula behind; 

 aal and aa2, first and second occipital arches of segments 6 and 7; om, oculomotor nerve; prf, profundus nerve; scl, sclerotome 

 of segmentlO; spl, vestigial dorsal root and ganglion of first spinal nerve; sp2, second spinal; t, trochlear nerve; tr, trigem- 

 inal nerve; v, complex root of vagus nerve; vgl, vestigial dorsal root and ganglion of segment 7; vc, ventral coelom extending 

 up each visceral bar; vr, ventral nerve root of segment 6, supplying second metaotic myotome and hypoglossal muscle; 



vs, Umit of visceral region. 

 After Goodrich, 1918.2, Text-fig. 1. 



For the adult Chlamydoselachus, the chief cranial nerves are represented in my 

 Figure 29, plate VII. The roots of the cranial nerves are shown in Figures 13 and 14, 

 plate IV, and in Text'figure 119. For comparison, I have inserted a figure showing the 

 cranial nerves of Squalus (Text'figure 120). My principal illustration of the cranial 

 nerves of Chlamydoselachus (Figure 29, plate VII) is complicated by a diagram of the 

 lateral line system of sensory canals. Hawkes, throughout her work on Chlamydoselachus, 



