THE ORGANS OF THE INNER GERM-LAYER. 287 



parallel branchial leaflets, which are provided with the greatest 

 abundance of capillary blood-vessels. In this manner the most 

 anterior section of the alimentary canal, which lies immediately 

 behind the head, has become converted into an organ of respiration 

 adapted to life in water. 



The important differentiation of the alimentary canal into an anterior re- 

 spiratory chamber and a following nutritive region is possessed by Vertebrates 

 and Amphioxus in common with certain Invertebrates (Tunicates and 

 Balanoglossus). 



Likewise in the case of the higher (amniotic) Vertebrates both 

 inner and outer visceral furrows, together with the visceral arches 

 separating them, are, as has already been stated, formed ; but here 

 they are never developed into an actually functioning respiratory 

 apparatus ; they belong consequently in the category of rudimentary 

 organs. Upon the mucous membrane there arise no branchial leaflets; 

 indeed the formation of open clefts is not always and everywhere 

 achieved, since the thin epithelial closing membranes between the 

 separate visceral arches are preserved at the bottom of the externally 

 visible furrows. Upon this point, however, the opinions of the 

 investigators who have been engaged in the study of the throat-region 

 in late years are very dissimilar. Whereas His, BORN, and KOLLIKER 

 maintain that the closing plate does not as a rule rupture, FOL, DE 

 MEURON, KASTSCHENKO, LIESSNER, and others find that at least the 

 first two or three visceral clefts are temporarily open. The opening 

 takes place to a greater extent in Reptiles than in Birds and 

 Mammals, where it remains limited to a small territory. In the most 

 posterior visceral pockets there can be no breaking through, because 

 they are not as deep, and the closing plate is therefore thicker and 

 contains also a layer of connective tissue. The conditions in Reptiles 

 and Mammals, as well as the differences in the number of visceral 

 arches, to be mentioned directly, express separate stages in the 

 process of regressive metamorphosis, to which the whole visceral 

 apparatus in the vertebrate series has been subjected. 



The number of visceral clefts which actually appear in the separate 

 classes of Vertebrates is variable. The greatest number is en- 

 countered among the Selachians, where there may be as many as 

 six (fig. 155), in a few species indeed seven or eight. In Teleosts, 

 Amphibia, and Reptiles the number sinks to five. In Birds, 

 Mammals, and Man (figs. 154 and 157) only four arise. We can 

 therefore say in general that from the lower to the higher Vertebrates 

 a reduction has taken place in the number of visceral clefts which 



