144 THE ANCESTRY OF VERTEBRATES 



the lateral part of the dorsal branches (ramus lateralis) and 

 further the lateral and visceral part of the ventral branches 

 (rami post- and praetrematici) of the dorsal spinal nerves 

 behind it. Evidently under the influence of the vagus the 

 anterior spinal ganglia behind it are not so well developed 

 in Gnathostomes as those further back. In none of the 

 publications on the Cyclostomes is any process of this 

 nature referred to. The first spinal ganglion behind that 

 of the primary vagus fuses with it ("spinalartiger Vagus- 

 anhang", HATSCHEJC, 1892, p. 156). 



For the first time we find in Petromyzon a cartilaginous 

 skull, comparable to the head caitilage of Cephalopodsand 

 arising, like the latter, in close connection to the central 

 part of the nervous system and the main sense-organs. Its 

 extension is less than in higher Craniates and comprises, 

 besides the prostomium, only two segments (HatsCHEK, 

 1892, p. 159). This conception results from the observation 

 that the cartilaginous skeleton does not extend further back 

 than the auditory capsule and that the ganglion of the 

 glossopharyngeus and the vagus is situated behind it. 

 However, the considerable size of the auditory capsule 

 causes the latter to extend backwards into the anterior 

 two post-otic segments in which the internal parts 

 of the myotomes cs a consequence atrophy, while their 

 myocommata are attached to the auditory capsule (cf. fig. 

 29). The first neural arch is the one situated in the myo- 

 comma between the third and the fourth post-otic somite 

 (fig 29, occ. a.). Membranous walls connect the cartila- 

 ginous skull with the first neural arch. Through this 

 membranous part of the skull the glossopharyngeus and 

 the vagus pass. 



We shall call a skull as found in Petromyzon an i n- 

 complete (partly membranous) palaeocranium. 



Finally it may be observed that the hypophysis originates 

 from the ectoderm in front of the mouth involution. 



Amphibian head. — Next to the Cyclostomes to which 

 they show a close resemblance in their earliest development 

 come the Amphibians in which the hypophysis still originates 

 in front of the mouth involution, while this group stands 

 also nearest to Petromyzon in the structure of the brain, 

 especially of the metencephalon. The roof and the hinder, 

 membranous, part of the Cyclostome skull, between the 

 anterior, cartilaginous, region and the first neural arch, has 



