Comparative Development. 27 



The formation of the bones, muscles, &c. , is effected by the differen- 

 tiation of cells of the skin-fibrous layer. 



The embryo big toe in man is thumb like, as it is in the adult ape. 



The tail is formed simultaneously with the limbs, but at first, more 

 rapidly, so that in tne human and other vertebrate embryo at an early 

 stage it is considerably longer than the hind limbs. 



The anal opening is formed the same as the mouth, by the indenta- 

 tion of the skin layer from the outside as a blind sac growing inward 

 until its end comes into contact with the blind end of the intestine when 

 the end walls are absorbed and the two cavities become a continuous one. 



FIG. 31. The small fig. represents the human egg the 12th 

 or 13th day not opened natural size. 



The large fig. is the human egg about the 21st day, natural 

 size, opened, showing embryo with its yelk sac within. It 

 is already attached to the inside coat of the egg by a short 

 navel cord, and some of the gill arches, the rudiments of the 

 fore limbs, and of the ear, eye, and heart, are begun. 



(Allen Thompson.) 



CHAPTER IV. 



COMPARATIVE DEVELOPMENT. 



At the end of four weeks, of human embryonic age, the development 

 has passed through some of the most important stages. Thus the heart 

 has reached its four chambered stage, the lungs are commenced, the 

 primitive kidneys in full operation, the liver begun, the intestinal cavit}- 

 open from mouth to anus, both of which openings are already formed, 

 the short, paddle- like limbs are started, the tail is quite prominent, and 

 the jaw processes and the eye and ear indentations are commenced. 



FIG. 32. 



FIG. 32. Human embryo 7 weeks. (Magnified about 7 diameters.) after Quain. 

 ss Indications of vertebras. 1, 2, 3. Brain bladders. A:. Gill arches for a younger 

 embryo e -Ear. a, Z.-Arm and leg. .-Tail. w.-Umbihcal opening. 



