22 ILLINOIS BIOLOGICAL MONOGRAPHS [22 



end and the occipital arch there is a small triangular piece of cartilage, the 

 claustrum of the Weberian apparatus. Sagemehl (1885) regarded this in the 

 Characinidae as the first true neurapophysis, and maintained that the nerve 

 which originally passed between the claustrum and the scaphium has been 

 suppressed in those groups having a Weberian apparatus. The scaphium 

 was homologized to the second neurapophysis. 



The second pair of neurapophyses lie a short distance posterior to the 

 scaphia and are separated from them by a pair of rather wide foramina through 

 which the second pair of postvagal nerves issue (Figs. 12, 35). The ventral 

 ends of these neurapophyses are concave and are closely appHed to the noto- 

 chord, in contradistinction to the rounded ends of the scaphia. Above the 

 dorsal ends of this second pair of neurapophyses and the claustrima, the pos- 

 terior end of the occipital arch has narrowed to a small process which is 

 inserted into the anterior face of the third neural arch (Fig. 12). The posterior 

 and ventral ends of this neural arch descend behind the second pair of neura- 

 pophyses, leaving a wide foramen in the wall on each side. The third post- 

 vagal nerve (Fig. 35) passes out through this foramen, nearer to the second 

 neurapophysis than to the third. 



Briefly, the skeletal and nerve elements alternate with each other in this 

 region, just as they do farther back in the body (Fig. 35). None of the post- 

 vagal nerves are actually included within the cranium at this stage, as the 

 dorsal and ventral parts of the occipital region have not as yet united posterior 

 to the first pair. The dorsal surface of the occipital arch does not show any 

 segmentation and ventrally it is continuous with the parachordaUa posterior 

 to the passage of the vagus nerve and the otic capsule (Fig. 21). The para- 

 chordal cartilages do not extend posteriorly beyond the passage of the first 

 post-vagal nerve and there is no cartilage lateral to the notochord until the 

 ventral ends of the third neural arch are reached. 



There are four distinct muscle segments between the posterior end of the 

 otic capsule and the ventral end of the third neural arch on each side (Fig. 35). 

 The more anterior are dorsal and oblique to the posterior, which extend in 

 under their ventral ends (Figs. 12, 13). The first myotome is very short and 

 projects into the shallow temporal fossa, lateral to the dorsal half of the occipi- 

 tal arch (Fog. 12). The second starts ventral to the first, and lateral to the 

 passage of the first postvagal nerve. It is separated from the cartilage of the 

 occipital arch by a wide space filled with loose connective tissue. The third 

 myotome comes in below the second, lateral to and above the second post- 

 vagal nerve, and the fourth is lateral to the anterior projection of the neural 

 arch of the third vertebra. Distinct myosepta are present between all of these 

 segments. 



All the post-vagal nerves have both dorsal and ventral roots, but the first 

 is the only one in which the ganghon of the dorsal root lies within the cavum 

 spinalis. The gangha of the others all lie external and lateral to the side walls of 



