INTESTINAL TEACT OF MAMMALS. 



521 



communication with the colon. Here, again, it is obvious that the condition is easy to 

 understand if it be supposed that in the ancestral form the ileum opened into the colon 

 between a pair of symmetrically placed caeca. 



The diagrammatic outlines in fig. 50, p. 522, show what appear to be the obvious 

 interpretation of the Mammalian caecum. In the upper figure the primitive paired 

 condition is shown ; in the lower figure the only modification is the suppression of 

 one member of the pair. 



Eig. 49. 



lleo-cceeal regions in Gallithrix cuprea (I), Cebus fataellus (II), and Macacus rhesus (III). 

 C in I. Cut end of cajcum. 

 C.A. Projecting tubular aperture of caecum to colon. 

 Other letters as in fig. 48. 



I conclude, then, that the caeca in Birds and Mammals are homologous. Primitively 

 they are paired outgrowths of the hind-gut situated near the point where the primitive 

 straight rectum joins what is known as the pendent loop. In Birds they appear in 

 the adult and in the embryo, nearly at the point where the pendent loop joins the 

 straight rectal portion, this position being due to a phylogenetic shortening or failure 

 of expansion of the hind-gut. In Mammals they lie some way on the posterior limb of 

 the dorsal pendent loop, and this position is due to an expansion of the hind-out. In 

 Birds, typically, both caeca develop symmetrically, but there are cases where only one 

 develops. In Mammals, typically, one caecum degenerates, but there are cases where 

 both are developed symmetrically, and very numerous cases where one remains in a 

 more or less vestigial condition. The irregularity of the distribution of symmetrically 



