2IO 



CHORDATE ANATOMY 



Lateral Trunk Muscles. The lateral trunk muscles of man develop 

 from myotomic segments which first appear in the fourth week of onto- 

 genesis, and by the end of the second month have increased to nearly 



SCLEROTOME- 

 HYPOCHORDA 



NEURAL CREST 



NEURAL TUBE 



■ jil^EPIMERE 



/wiL 'SOMATIC MOTOR NERVE— 

 r^^^\^~--— NOTOCHORD-''" 



MESOMERE-y^' 



ECTODERM j ^ 



— ENDODERM 



CRMATOME 

 MYOTOME 

 SCLEROTOME 

 HYPOCHORDA 



HYPOMERE 



COELOM 



SUBINTESTINAL, 

 BLOOD VESSEL 



A. HEAD B. TRUNK 



Fig. 198. — 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. 



EVTREMITV 



SOMATIC MUSCLE 



INTESTINE 

 ENDODERM 

 COELOM 



NEURAL CREST 



SOMATIC MOTOR NERVE 

 SPINAL CORD 

 SPINAL GANGLION 

 RAMUS DORSALIS 

 NOTOCHORD 

 CENTRUM 

 NEPHROTOME 

 CORIUM 

 VISCERAL MUSCLE 

 RAMUS VENTRALIS 



•r /" DERMATOME 

 MYOTOME 

 SCLEROTOME 

 PARIETAL MESODERM 

 ISCERAL MESODERM 



Fig. 199. — A stereogram of the trunk region of a vertebrate embryo, based upon 

 elasmobranch embryos. The figure shows an earlier stage of development on the right 

 side, a later stage on the left. The extension of the myotome to form the lateral trunk 

 musculature is shown. The lateral trunk musculature of the ventral half of the body- 

 wall thus arises as a secondary invasion. (Redrawn after Braus.) 



forty pairs. The original metamerism of the myotomes, which persists 

 even in the adults of the lower vertebrates, becomes largely lost in adult 

 man and mammals as the result of a number of processes among which 



