160 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



of the succeeding clefts, so that the spiracular pseudobranch would 

 on a strict interpretation of the germ-layer theory not he serially 

 homologous with the other gills. 



In the present writer's opinion, as already indicated, such obser- 

 vations upon the first origin of organs which develop in the region 

 of the blurred boundary between two layers are not to be taken as 

 affording evidence of any serious importance in regard to the 

 morphological nature of such organs. Greater weight however seems 

 due to evidence obtained from cases where the hrst traces of gill 

 lamellae are visible at a period before the bounding membrane of 

 the cleft is ruptured, when the cleft consists still of two distinct 

 pouches— one ectodermal, the other endodermal— separated by a 

 still complete partition. Such is the case in Acipenser and Goette 

 shows that in this case the lamella-rudiments arise outside the 



partition from what is undoubtedly an 

 ectodermal surface (see Fig. 90, g.l). 



The same discussion extends to the 

 general lining of the cleft — as to how 

 much of the lining of the adult cleft is 

 ectodermal and how much endodermal. 

 Goette and Moroff (1902) hold that only 

 the portion of the cleft in the imme- 

 Fio.90.-Horizontai section through dkte neighbourhood of its pharyngeal 



branchial region of young Aci- . . ° . .. r ■ r ° , 



penser showing the ectodermal opening is to be regarded as endodermal, 



origin of the gill lamellae. (Atter all the rest being ectodermal. But here 



Goette, 1901.) again in view of the blurred character 



oo, aortic arch ; g.i, rudiment of gin ot - tne boundary between the two layers 



lamella; Hii, hvoid arch ; m>, operculum ; ., , ,, (1j , , t , 



pa, cavity of pharynx. it seems hardly profitable to speculate 



on the matter. 



In certain fishes the gill-lamellae are for a time prolonged out- 

 wards into long threads which project through the cleft opening into 

 the surrounding fluid. Such is the case in the embryos of Elasmo- 

 branchs, in which it is only the lamellae upon the posterior face of 

 each arch that become prolonged, those on the anterior face not 

 projecting beyond the edge of the septum. Eventually the pro- 

 jecting part of the filament disappears while its attached basal 

 portion becomes the definitive lamella. In a few Teleosts a similar 

 temporary modification of the lamellae takes place — perhaps the 

 best example being Gymnarchus (Budgett, 1901 ; Assheton, 1907. 

 See Fig. 199). 



Evolutionary History of the Branchial Eespiratory 

 Organs. — As regards the early evolutionary history of these branchial, 

 respiratory organs one very generally accepted view looks upon the 

 visceral clefts as being the most primitive, the internal gills as 

 having developed next, and the external gills as being due to 

 secondary extension of respiratory tissue outwards from the clefts. 

 It seems however, bearing in mind what we now know regarding the 

 development and distribution of external gills, at least equally if not 



