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166 THE ORIGIN OF VERTEBRATES 
teaches that all animals above the Protozoa are derived from a form 
which by invagination of its external surface formed an internal 
cavity or primitive gut. From pouches of this gut other cavities 
were said to be formed, called ccelomic cavities, and thus arose the 
group of ccelomatous animals. To speak of the developmental history 
of animals in terms of spaces; to speak of the atrophy of a cavity 
as though such a thing were possible, is, to my mind, the wrong 
way of looking at the facts of anatomy. It resembles the description 
of a net as a number of holes tied together with string, which is not 
usually considered the best method of description. 
There are two ways in which a series of pouches can be formed 
from a simple tube without folding, either by a thinning at regular 
intervals of the original tissue surrounding the tube, or by the 
ingrowth into the tube of the surrounding tissue at regular intervals, 
thus— 
Fic. 67.—D1aGRAMS TO SHOW THE TWO METHODS OF POUCH-FORMATION, 
A, by the thinning of the mesoblast at intervals. B, by the ingrowth of mesoblast at 
intervals. Ep., epiblast; Mes., mesoblast; Hy., hypoblast. 
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 ease (B) the formation of a pouch is 
