THE EVIDENCE OF THE RESPIRATORY APPARATUS 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 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 Ammoccetes 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 ccelomic 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 
