1 66 



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 coelomic cavities, and thus arose the 

 group of cceloinatous 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 — 



A 



Ep - 

 Mes- 



ODOOOQQQQQDCDC03CG00aQO0Q3 



\ it; t 1 1 rr- 



OQSDODDaDCDDDaDQOSaOSaDDODBa 



'SEGMENT' ; 



araQoaQQaaQDaQQbgoQoaaoDOQOSDO 



B 



Ep- 



Mes- 



% 



1 



CDCKBCCCDDDBODCOBGDBDOaDDD. 





DDanuuoauauaaDnanaaD 



QGCOEBDaCDa00aBQDDBO0DQ3BQQag 



Fig. 67. 



1 z 



-Diagrams to show the two methods of Pouch-formation. 



A, by the thinning of the mesoblast at intervals. B, by the ingrowth of rnesoblast 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 



