THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 67 



brought about in consequence of the ingrowth of the mesoblastic 

 tissues at intervals ; here, although the end-result is the same as in 

 the first case, the pouch-formation is only secondary, the true 

 branchial unit is the mesoblastic ingrowth. 



The evidence all points directly to the second method of forma- 

 tion. Thus Shipley, in his description of the development of the 

 lamprey, says — 



"The gill-slits appear to me to be the result of the ventral 

 downgrowth of mesoblast taking place only at certain places, these 

 forming the gill-bars. Between each downgrowth the hypoblastic 

 lining of the alimentary canal remains in contact with the epiblast ; 

 here the gill-opening subsequently appears about the twenty-second 

 day." 



Dohrn describes and gives excellent pictures of the growth of 

 the diaphragms, as the Aminoccetes grows in size, pictures which 

 are distinctly reminiscent of the corresponding illustrations given 

 by Brauer of the growth of the internal gills in the scorpion embryo. 



Another piece of evidence confirmatory of the view that the 

 branchial segments are really of the nature of internal appendages, 

 as the result of which gill-pouches are formed, is given by the presence 

 in each of these branchial bars or diaphragms of a separate ccelomic 

 cavity. From the walls of this cavity the branchial muscles and 

 cartilaginous bar are formed. 



Now, from an embryological point of view, the vertebrate shows 

 that it is a segmented animal by the formation of somites, which 

 consist of a series of divisions of the ccelom, of which the walls form 

 a series of muscular and skeletal segments. In the head-region, as 

 already mentioned, such ccelomic divisions form two rows — a dorsal 

 and a ventral set. From the walls of the dorsal set the somatic 

 musculature is formed. From those of the ventral set the branchial 

 musculature. From the latter also the branchial cartilaginous bars 

 are formed. Thus Shipley, in his description of the development 

 of the lamprey, says: "The mesoblast between the gills arranges 

 itself into head-cavities, and the walls of these cavities ultimately 

 form the skeleton of the gill-arches." 



Similarly, in the arthropod, the segments in the embryo are 

 marked out by a series of co?loniic cavities and Kishinouye has 

 described in Limulus a separate ccelomic cavity for every one of 

 the mesosomatic or branchial segments, and he states that in Arachnida 



