THE NEW MOUTH. 253 



At just what stage in the closing of the medullary plate, the mouth ceased to 

 communicate with the exterior cannot, at present, be determined. The final 

 closure was no doubt hastened by the crowding of the optic ganglia upward and 

 backward over the oral region. This would leave the broadest part of the primi- 

 tive brain, i.e., the region of the fourth ventricle and the rhomboidal sinus, wide 

 open; and in this region, which is now covered only by the choroid plexus, there 

 remained, probably for some time, an opening through which the old stomodaeum 

 could communicate with the exterior. (Figs. 3, 46, D.) 



Thus, with the knowledge acquired after the event, we may look back a few 

 million years, and trace with our mind's eye, the slow, inevitable approach and 

 consummation of the most momentous event in organic evolution. 



II. THE NEW MOUTH. 



A group of animals in which the natural growth of one essential part inevita- 

 bly eliminates some other part equally essential, is doomed to extinction, unless 

 among the' organs already at hand a radical redistribution of functions is possible 

 at the moment the critical period arrives. No organ was ever created, we may be 

 sure, to meet an organic crisis in the future, or was ever produced, de novo, at the 

 demand of a present necessity. We are bound to assume that unless a suitable 

 organ stands ready to do the work of the one that has been eliminated, no way 

 out of the difficulty is possible. 



Such was the situation when the great crisis in the evolution of vertebrates 

 was at hand; either the evolution of the brain must cease, or a new entrance to the 

 midgut must be established elsewhere. The alimentary organs proved most 

 pliable. But how significant it is that the momentum of nerve growth should so 

 dominate the growth of other organs, and the form of the nervous system so 

 modify that of the whole body! 



No doubt one important factor in the competitive development of brain and 

 stomodaeum is the increasing volume of the yolk sphere; for the presence of more 

 yolk postpones to a later and later embryonic stage the time when the stomodasum 

 becomes functional, and thus allows the precociously developing nervous system to 

 undergo its early stages of development, unmodified by the action of the stomo- 

 daeum in feeding. 



In the arthropods, a very old organ, the "dorsal organ," or cephalic navel, 

 having originally a very different function from that of alimentation, stood ready 

 to take the place of the old mouth that was being slowly eliminated. Its presence 

 alone made the existence of the vertebrates, as we know them, a possibility. 



We have shown in a previous chapter, that the "dorsal organ," in part at 

 least, is the product of the mechanical conditions created by apical growth on a 

 spherical surface. It is, as it were, a vortex center, toward which all the adjacent 

 organs converge. It exists, either actually or potentially, on the anterior haemal 

 surface of all arthropod embryos, its definitive position being controlled, in any 



