OUR FACE FROM FISH TO MAN 



sea-cucumbers, and also of the acorn-worm Balan- 

 oglossus. The mouth of these forms is the original 

 mouth of the primitive gut or digestive tract. 



There is evidence from embryology that the 

 mouth of the vertebrates is a compound structure 



pmea? 



notoc/2orcC 



nasal hvpophysismoufM-cairiiy 

 pit 



B 



mouth- 

 cavity 



Aeart 



Fig. 56. Inner and Outer Mouth Pouches in Embryo Verte- 

 brates: Larval Lamprey (A) (after Minot); Embryo 

 Rabbit (B) (after Mihalcovics). 



For details, see p. xxv. 



formed from the union of a down-pocketing of the 



outer layer or ectoderm, meeting a pouch growing 



out from the primitive gut. These inner and 



outer mouth pouches in the early embryos of 



lampreys, sharks and higher vertebrates, are 



similar to the inner and outer pouches that give rise 



to the gill openings, with which indeed they are 



supposed to be homologous. Moreover Stensio 



has recently shown that in the cephalaspid 



94 



