Ill 



EXTERNAL GILLS 



155 



deeper portions a rich deposit of yolk, so as to look exactly like the 

 yolk-laden endoderm. Greil explains this appearance by supposing 

 that true endoderm cells actually 

 spread outwards and replace the 

 deep layer of the ectoderm, so that 

 the external gill-rudiment would be 

 partly eudodermal 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 which 

 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 Hypogcophis (Fig. 87) or 

 in Lepidosiren (Fig. 200). In 

 Lepidosiren there are present four 

 upon each side of the body. At 

 first the four are quite independent 

 of one another but as development 

 goes on they become raised upon a 

 common base so as to give the 

 appearance of a single organ with 

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



The distribution of true external gills amongst the main groups 

 of Vertebrates is shown in the following table : 



Fft;. 87. — Hypogeophis embryos showing 

 development of the external gills. 

 (After Brauer, 1899.) 



(i.ij, external gill ; H, hyoid arch ; olj, 

 olfactory organ. The rounded knobs seen 

 projecting in B from the hyoid arch, and also 

 from the mandibular arch in front of it, are 

 possibly external gill rudiments which do not 

 go on with their development. 



?.' = Vestigial. 



In those animals in which they are well developed the external 

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



