LATERAL CANAL SYSTEM OF SELACHIANS 43 



bud. It appears as a narrow, slightly elongated cord, and the 

 epidermal infolding at its growing end has as.sumed the char- 

 acter of a little pouch with its crescent-shaped opening du-ected 

 forwards. The shape of the fold is explamed by the fact that 

 the central part is carried backwards by the rapidly elongating 

 thickened cord of ectoderm, while the upper and lower margins 

 are retarded by the neighboring common ectoderm with which 

 they are continuous. 



The root of the lateralis ganglion has become more distinct 

 and its superficial attachment to the brain wall is more widely 

 separated from the vagus roots. From the posterior limit of 

 the ganglion, which is sharply defined, a large bundle of nerve 

 fibers passes to the inner sm'face of the lateral sensory cord. 



Eighteen millimeter stage. In this embryo (fig. 37) all of 

 the sensory cords have greatly elongated. The supraorbital 

 reaches almost to the anterior margin of the eye ; the infraorbital 

 has attained an equal growth below the eye, and both cords 

 appear as narrow ridges occupying perceptible groo^'es. For 

 their greater length the nerves which supply these cords lie 

 inunediately subjacent; owdng to the growth of the embryo, 

 however, the proximal ends of the nerves now occupy a posi- 

 tion slightly beneath the integument. 



Ectodermal folds appear at the growing ends of the head 

 cords, but their walls degenerate rapidly and for that reason 

 they do not assume the character of an elongated tunnel such 

 as develops over the lateral cord. 



The lateral cord of this stage reaches as far caudad as the 

 middle of the pectoral fin (fig. 37). It shows marked lateral 

 convexity and lies in a slight groove. The epidermal tunnel 

 is rather prominent. This primitive tunnel must not be con- 

 fused ^\ith the lateral canal which arises later. 



The gro\^dng end of the lateral cord is greatly enlarged wdth 

 dividing cells and it is quite evident that the superficial ecto- 

 dermal tunnel is formed by the ordinary ectoderm as it is pushed 

 up by the growing sensory cord. The layer of ectoderm which 

 was originally continuous with the lateral sensory canal anlage 

 is carried along with the growing end, thus giving two layers to 

 the outer wall of the ectodermal tunnel (figs. 38, 36). 



