238 CHARLES R. STOCKARD 



cells in the gut wall of this locality. The pancreas in different 

 vertebrate groups illustrates the phylogenetic steps in the devel- 

 opment of a budding outgrowth from the wall of a linear canal. 



The entire alimentary tract in the lowest vertebrates was no 

 doubt originally a simple tube and the lateral outgrowths or buds 

 are the highly specialized organs that have become so excessively 

 developed as to necessitate their separation from the tube. Thus 

 in phylogeny as well as ontogeny of the vertebrate gut it would 

 look as though the primary growth was linear and its complexity 

 has been added by lateral buds and offshoots. 



The above being the general state of affairs in the development 

 of the foregut, we come now to the point of experimental impor- 

 tance in the dynamics of these organ-forming processes. And 

 that is, that each of the organs derived from the entodermic wall 

 is in its critical or sensitive stage at the moment of its outgrowth, 

 or at the time of the excessive cell proUferation in its region of the 

 wall. Should any condition be introduced which would lower the 

 general developmental rate, that organ will be most affected which 

 happens at the time of the arrest to be in or nearest its critical 

 moment. Thus an arrest during very early development will 

 inhibit the growth of the mandibular pouch and through its mal- 

 formation distort the formation of the mandibular arch, causing 

 deformed and strangely developed mouths (see figures of the 

 deformed fish). Since the hyoid and other branchial pouches 

 arise so nearly at the same time as the mandibular pouch, they, 

 with later accompanying structures, are likewise almost invaria- 

 bly deformed along with deformities of the mandibular structures. 

 Such deformities as these may, however, exist in individuals with 

 perfectly normal stomachs, livers, etc. In these cases, a normal 

 or fair rate of development had again been established before the 

 critical moment in the origin of the latter organs had arrived. 



It would thus seem possible that an experimenter might inhibit 

 at will the rate of development during particular intervals and 

 thereby succeed in suppressing and deforming the mouth and 

 branchial structures and leave the more caudally situated organs 

 uninjured. Or, reverse the experiment and obtain normal mouth 

 and gill structures in an embryo with suppressed and underde- 



