TIM: ORGAHfl 01 i in: H M-I AVER. 287 



parallel branchial leaflets, which are provided with the greatest 

 abundance of capillary bleod-ves-els. hi this manner tin- most 

 anterior Section of the alimentary canal, which lie, iininiMh 

 behind tin- head, has 1 ecome converted into an organ of n-.-jui 

 adapted to life in water. 



Tin- important ililTrivntiation of the alimentary canal into an ant.-ri 

 spiratory chamber and a following nutritive r.-u r i"n is po. 

 and Anipliioxus in common with certain Invertebrates (Tunicates and 

 Halanoglossus). 



Likewise iii the case of the higher (aiuniotic) Vertebrates both 

 inner and outer visceral furrows, together with the visceral arch. - 

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

 they are never developed into an actually functioning respiratory 

 apparatus : they In-long 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 tho 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 t ln-o.it i 

 in late years are very dissimilar. Whereas His, BoBN, and KOI.UKKU 

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

 MKri;\. K ASTSCIIENKO, LiESSNEB, and others find that at least the 

 iii>t two or three visceral clefts are temporarily open. The opening 

 takes place to a greater extent in Reptiles than in I'.irds and 

 M animals, 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 then fore thicker and 

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

 and Mammal-, 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. 



TJt>' number of visceral clefts wfiirJt artmtUii a^^ar in the separate 

 classes of Vertebr variable. The greatest numb- 



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

 six (fig. 155), in a few Bpeciefl indeed seven or eight. In Tel. 

 Amphibia, and Reptiles the number sinks to five. In I'.inK. 

 Mammals, and Man (figs. 154 and lf>7) only four arise. We can 

 tfierefore say in genera/ tluit from the Imr^r to tic 

 a reduction has taken plac*: in the number of visceral clefts which 



