In the first case (A) the formation of a pouch is the significant act, and therefore the branchial segments might be expressed in terms of pouches. In the second case (B) the formation of a pouch is {167}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 formation. 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 Ammocoetes 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 coelomic 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 coelom, of which the walls form a series of muscular and skeletal segments. In the head-region, as already mentioned, such coelomic 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 coelomic cavities and Kishinouye has described in Limulus a separate coelomic cavity for every one of the mesosomatic or branchial segments, and he states that in Arachnida {168}the segmental coelomic cavities extend into the limbs. These cavities both in the vertebrate and in the arthropod disappear before the adult condition is reached. The whole evidence thus points strongly to the conclusion that the true branchial segmental units are the branchial bars or diaphragms, not the pouches between them.