Ill 



EXTERNAL GILLS 



155 





deeper portions a rich deposit of yolk, so as to look exac.tly like the 

 yolk-laden endoderm. (ireil explains this appearance by supposing 

 that true endoderm o-lls actually 

 spread out \\ards and replace the 

 deep layer of the ectoderm, so that 

 the external gill-rudiment would be 

 partly endodernial in its nature. 

 There is however no definite evi- 

 dence of any such process taking 

 place and the present writer would 

 interpret the appearances as mean- 

 ing simply that the ectoderm cover- 

 ing the external gill - rudiment 

 becomes thickened, and stores up 

 a supply of yolk in its deeper layers, 

 as a physiological preparation for 

 the active processes of growth whie.h 

 are about to take place as the 

 external gill rapidly increases in 

 length. In this he agrees with 

 Marcus (1908). 



The general appearance of the 

 developing external gills is well 

 seen in Hypogeophis (Fig. 87) or 

 in Lepidosiren (Fig. 200). In 



Lepidosiren there are present four FIG. 87. Hypogeophis embryos showing 

 upon each side of the body. At 

 first the four are quite independent 

 of one another but as development 



goes 011 they become raised Upon a projecting in B from the hyoid arch, and also 



common base so as to give the 



/ * 1 .! LMjaoiMiv C.VUGJ. lien Km AIAVAIIIH.* 



appearance of a single organ with go on with their development 

 four branches (Fig. 200, B-E). 



The distribution of true external gills amongst the main groups 

 of Vertebrates is shown in the following table : 



development of the 

 (After Brauer, 1899.) 



external gills. 



xternal gill; 77, hyoid arch; olf, 

 olfactory organ. The rounded kim! 



from the mandibular arch in front of it, are 

 possibly external -ill rudiments which do not 



In those animals in which they are well developed the external 

 gills are, for a time the main functional breathing organs. They are 



