THE -DEVELOPMENT OF THE FROG 281 



A comparison of the development of the frog with that of 

 Amphioxus shows many points of resemblance in the earlier 

 stages, but in the later stages there are numerous differences, 

 attributable to the different grade of organisation and greater 

 complexity of structure of the mature frog. In a comparison 

 of the development of the frog with'that of higher vertebrates 

 the resemblances would be more obvious in later stages, in some 



cor 



vit 



Fig. 70 



Section through the eye of a tadpole at the time when the operculum is 

 forming, much magnified. ck, choroid ; cor, mesoblast cells which will give 

 rise to the cornea ; dm, Descemet's membrane ; c/>, pigmented external 

 epithelium ; L^ lens ; nies, branched mesoblast cells ; op.n, optic nerve ; pch, 

 pigmented epithelium of the choroid ; R , retina ; rd, rods and cones pulled 

 away from the pigmented epithelium of the choroid by contraction of the 

 preparation ; vit, cells of the vitreous humour. 



organs, such as the eye, amounting almost to identity. Thus 

 we should find throughout the vertebrate series that the 

 axial skeleton is first formed as a notochord, which subse- 

 quently becomes surrounded with a cartilaginous sheath, and 

 this in turn is supplanted by bony rings or vertebrae. In the 

 skull we should find a cartilaginous cranium, formed from 



