DEVELOPMENT. 789 



From the Mesublast. The urinary and generative organs (except the 

 epithelium of the urinary bladder and urethra). 



All the voluntary and involuntary muscles of the body (except the 

 muscular fibres of the sweat glands). 



The whole of the vascular and lymphatic system, including the 

 serous membranes and spleen. 



The skeleton and all the connective tissues and structures of the body. 



From the Hypoblast. The epithelium of the alimentary canal from 

 the back of the mouth to the anus, and that of all the glands which 

 open into this part of the alimentary tube. 



The epithelium of the Eustachian tube and tympanum. 



The epithelium of the bronchial tubes and air sacs of the lungs. 



The epithelium lining the vesicles of the thyroid body. 



The epithelial nests of the thymus. 



The epithelium of the urinary bladder and urethra. 



It remains now to consider in succession the development of the 

 several organs and systems of organs in the further progress of the 



gjL , 



tricle"; L, lensTcVsYchoroidYf sfit; ~Cen ^V~ auditory vesicle; s m, superior maxillary process; 

 IF, 2F, etc. , first, second, third, and fourth visceral folds ; F, fifth nerve, sending one branch 

 (ophthalmic) to the eye, and another to the first visceral arch ; VII, seventh nerve, passing to the 

 second visceral arch : G P/i, glosso-pharyngeal nerve, passing to the third visceral arch ; P g, 

 pneumogastric nerve, passing toward the fourth visceral arch ; i v, investing mass ; c h, noto- 

 chord ; its front end cannot be seen in the living embryo, and it does not end as shown in the fig- 

 ure, but takes a sudden bend downward, and then terminates in a point ; Ht , heart seen through 

 the walls of the chest: M P, muscle plates; TF, wing, showing commencing differentiation of 

 segments, corresponding to arm, forearm, and hand ; H L, hind-limb, as yet a shapeless bud, 

 showing no differentiation. Beneath it is seen the curved tail. (Foster and Balfour.) 



embryo. The accompanying figure (fig. 485) shows the chief organs of 

 the body in a moderately early stage of development. 



The Vertebral Column and Cranium. The primitive part of 

 the vertebral column in all the vertebrata is the chorda dorsalis or noto- 



