6io 



COMPARATIVE ANATOMY 



and trunk segments as serially homologous. It has since been demon- 

 strated that visceral arches differ from ribs, both in development and in 

 relations to other parts, and hence are not homologous. Gegenbaur 

 called attention to the fact that the presence of membrane bones in the 

 skull makes it impossible to compare the skull with a vertebra. Like 

 Huxley, Gegenbaur counted nine segments in the head. These he 

 regarded as serially homologous with those of the trunk. 



Head Segments According to Gegenbaur 



Primary visceral 

 skeleton 



I St arch. 

 2nd arch 



3rd arch. 

 4th arch. 

 5th arch. 

 6th arch. 

 7th arch. 

 8th arch. 

 9th arch. 



Modified visceral 

 skeleton 



ist upper labial cartilage 

 2nd upper and ist lower 



labial cartilages 

 Mandibular arch 

 Hyoid arch 

 ist gill arch 

 2nd gill arch 

 3rd gill arch 

 4th giU arch 

 5 th gill arch 



Nerve 



^ Trigemini 



Ramus 21 



Ramus 3 ; 

 Facial 



Glossopharyngeal 

 Ramus, i vagi 

 Ramus, 2 vagi 

 Ramus, 3 vagi 

 Ramus, 4 vagi 



Embryological Phase. With the rise of the science of embryology 

 during the latter part of the nineteenth century, the head problem entered 



SCLEROTOME- 

 HYPOCHORDA 



V^ERMATOME 

 Ist-MTOTOME 



SCLEROTOME 

 HYPOCHORDA 



MESENTERY 



NEURAL CREST 



NEURAL TUBE 



EPIMERE 



'SOMATIC MOTOR NERVE 

 "^zES-V— -""^ NOTOCHORD-'" 



(.;;^^]T__— DORSAL AORTA 



MESOMERE 



ECTODERM 



— ENDODERM 



ENTERON 



HYPOMERE 



COELOM 



SUBINTESTINAL 

 BLOOD VESSEL 



A. HEAD B. TRUNK 



Fig. 504. — Diagrams of cross sections in A, head and B, trunk regions of an elasmo- 

 branch embryo showing the fundamental similarity of the two regions. The discovery 

 that the coelom of elasmobranch embryos extends throughout head and trunk and that 

 in this respect the two regions are alike was made by the English embryologist, Francis 

 Balfour. 



its third and last phase. The assumption which underlies the interpreta- 

 tion of embryological evidence, as applied to the head problem, is von 

 Baer's law, the fundamental law of biogenesis, that the development of the 

 individual recapitulates briefly the past history of the race. The truth 



