ONTOGENESIS 



237 



tern and eyes, provision for which is made very early, 

 at which time all vertebrate embryos look much 

 alike. All of the vertebrate embryos have long tails 

 (see the Romanes diagrams). The ventral surface 

 becomes closed by the concrescence of buds that form 

 the face and neck and the thoracic and abdominal walls. 

 The branchial region of all vertebrate embryos show 

 slits or folds where the gills of the fishes and batrachians 



FIG. 98. Diagrams illustrating arrangement of primitive heart and aortic 

 arches in the human embryo. By comparing these with the diagrams showing 

 the increasing complexity of the heart in phylogenetic development (Chapter 

 VII) it will be seen that in the development of the human heart the ontogeny, 

 repeats the phylogeny. (Modified from Allen Thomson.) 



are formed, though the mammalian embryos do not 

 have real gill clefts at this time. In all cases the limbs 

 first appear as buds that grow into shapeless excrescences, 

 which subsequently elongate, differentiate, and become 

 perfected, it being impossible to tell the final character 

 of these members for some time after they have first 

 appeared. 



The tail persists for a surprisingly long time in the 



