Development of the Fifth Day 277 



rived the following organs — the epidermis and 

 its appendages ; the lens of the eye ; the epi- 

 thelium of the cornea ; the olfactory chamber ; 

 the auditory organ ; the mouth, and the anus ; 

 the brain ; the spinal chord ; the retina ; the 

 optic nerve ; the nerve-fibres, etc. 



The entoderm almost from the first exhibits 

 two or three kinds of cells, found in the area 

 opaca, area pellucida, and embryo proper. 

 This layer also shows variations in thickness, 

 analogous to those seen in the ectoderm. 

 From the entoderm are developed the noto- 

 chord, and the epithelium of the entire res- 

 piratory and digestive tracts with their 

 appendages ; the epithelium of the mouth and 

 anus, however, are of ectoblastic origin, as has 

 been stated. 



The mesoderm early shows a differentiation 

 into several varieties, which, however, are 

 largely due to the method of grouping or the 

 varying degrees of condensation of the cells. 

 From the mesoderm are derived all the struc- 

 tures not mentioned in connection with the 

 other two germ-layers, such as muscle, bone, 

 blood, urogenital organs, etc. 



The most important changes in the develop- 

 ment of the fifth day are : 



