METAMERISM 



541 



derm is complete dorso-ventrally, including the hypomeric region of the 

 mesoderm. It becomes evident, therefore, that the suppression of segmentation 

 in the hypomeric area in the vertebrate embryo achieves a precocious result 

 which the embryo of Amphioxus reaches only at a later period of develop- 

 ment. Presumably in the vertebrate embryo, segmentation of the epimeric 

 mesoderm is retained because it serves a definite end, whereas segmentation 

 of the hypomeric mesoderm is deleted because it also leads to a necessary end 

 result in a direct manner. 



When applied to the developing head region, this procedure principle means 

 this: A primitive type of segmentation does tend to appear in the pre-otic 

 area as well as in the post-otic portion of the head, as indicated by the pre-otic 

 and post-otic somites, and secondarily there is developed a branchial metam- 



GASSEF1I4N GSNGLION OF NERVE T 

 METENCEPHl 

 GENICULATE GANGLION OF NERVE SH. 

 ACOUSTIC GANGLION OF NERVE TTTTT 

 MTELENCEPHAL 

 OTIC VESICLE 

 SUPERIOR GANGLION OF NERVE H 

 JUGULAR GANGLION OF NERVE I 

 PETROSAL GANGLION ( 

 NERV 

 NODOSE GANGLION 



Fig. 259. Chick embryo reconstruction of about 100 hrs. of incubation with special 

 reference to the nervous and urinary systems. See also fig. 336D. 



