CRANIAL SEGMENTS. 73 



segment is represented by the premandibular somite* an unpaired sac 

 with epithelial walls, immediately in front of the notochord (preoral 

 head cavity, vide p. 8). The walls of this sac give risa to all the eye- 

 muscles except the superior oblique and external rectus, and to mesen- 

 chyme. Its cavity vanishes, as do the cavities of all the cranial segments. 

 The nerves are the ram us ophthalmicus profundus, which develops from 

 the nerve crest immediately in front of the trigerninal and represents 

 the dorsal root, and the third nerve which represents the ventral root. 

 These two roots are both connected to the ciliary ganglion (see accoimtof 

 cranial nerves under Ela.smobranchii). 



The second segment is the mandibular somite which is dilated in its 

 dorsal muscle-plate region and extends ventrally to the lower end of the 

 mandibular arch (collar-somite, vide p. 7). The walls of this sac give 

 rise dorsally to the superior oblique muscle and ventrally to the mesen- 

 chyme and muscles of the mandibular arch. The nerves are the trigerninal 

 and the fourth, the latter being regarded as an abnormally situated anterior 

 root. The first two cranial somites were discovered by Balfour. 



The third and following segments are represented only by their dorsal 

 muscle-plate sections, the ventral portions being merged in the continuous 

 splanchnocoel (pericardial division). These segments may be regarded as 

 the anterior of the trunk series of Amphioxus. They do not apparently 

 communicate with the ventral splanchnocoel, which in this region under- 

 goes a pseudo-segmentation in consequence of the formation of the gill- 

 pouches. These pseudo-segments, or hyoid- and branchial- arch cavities, 

 open ventrally into the pericardium, of which they are a part, and were 

 taken by Balfour for the posterior cranial segments. The first of these 

 muscle-plates, which may be called, from its position, the hyoid myotome, 

 is better developed than the rest and gives rise to the external rectus 

 muscle. It was observed by Balfour. The next three, which were 

 discovered by v. Wijhe, are very faintly marked and transient and give 

 rise to no muscles, in correspondence with which fact may be noted the 

 absence of ventral roots. The next three segments (seventh-ninth) are 

 represented by well-developed muscle plates which persist and give rise to 

 muscles. The nerves of the hyoid segment (third) are the facial (dorsal 

 root) and sixth (ventral root). The nerves of the next three segments 

 are supposed to be represented by the auditory, glossopharyngeal and 

 vagus and are without ventral roots in correspondence with the absence of 

 myotome muscles. In the last three (or sometimes more) cranial seg- 

 ments, dorsal roots are present only in the embryo for a short time, but 

 ventral roots are developed, supplying presumably the myotome muscles 

 of this region and called the occipito-spinal nerves. These were mistaken 

 by Gegenbaur, who did not study the embryo and observe the transient 

 dorsal roots belonging to them, for ventral roots of the vagus. The 

 following table represents in brief the view of cranial segmentation 

 which has just been described. f 



* There is in some forms a pair of head cavities in front of the pre- 

 mandibular somite. These are sometimes in communication with and 

 developed as diverticula of the premandibular somite, but in Acanthias 

 they are said to be independent of it (J. B. Platt, Journal of Morvholoav . 

 5, 1891, p. 79). 



t The view here given takes no account of the scheme given on p. 77, 

 according to which the cranial nerves were originally tripartite, consisting 

 of dorsal, lateral and ventral roots. It was formulated before the modern 



