HANDBOOK OF PHYSIOLOGY. 



will be produced for them within. The primitive form of the extremity 

 is nearly the same in all Vertebrata, whether it be destined for swim- 

 ming, crawling, walking, or flying. In the human fo3tus the fingers are 

 .at first united, as if webbed for swimming; but this is to be regarded 

 not so much as an approximation to the form of aquatic animals, as the 

 primitive form of the hand, the individual parts of which subsequently 

 become more completely isolated. 



The fore-limb always appears before the hind-lirnb, and for some time 

 continues in a more advanced state of development. In both limbs alike, 

 the distal segment (hand or foot) is separated by a slight notch from the 

 proximal part of the limb, and this part is subsequently divided again 

 by a second notch (knee or elbow-joint). 



The Vascular System. At an early stage in the development of 

 the embryo chick, the so-called " area vasculosa " begins to make its 

 appearance. A number of branched cells in the mesoblast send out pro- 



FIG. 469. A human embryo of the fourth week, 3> lines in length. 1, the chorion; 3, part of 

 the amnion; 4, umbilical vesicle with its long pedicle passing into the abdomen; 7, the heart; 8, the 

 liver; 9, the visceral arch destined to form the lower jaw, beneath which are two other visceral 

 arches separated by the branchial clefts; 10, rudiment of the upper extremity; 11, that of the lower 

 extremity; 12, the umbilical cord; 15, the eye; 16, the ear; 17, cerebral hemispheres; 18, optic lobes, 

 corpora quadrigemina. (Muller.) 



cesses which unite so as to form a network of protoplasm with nuclei at 

 the nodal points. A large number of the nuclei acquire a red color; 

 these form the red blood-cells. The protoplasmic processes become 

 hollowed out in the centre so as to form a closed system of branching 

 canals, in the walls of which the rest of the nuclei remain imbedded. In 

 the blood-vessels thus formed, the circulation of the embryonic blood 

 commences. 



According to Klein's researches, the first blood-vessels in the chick 

 are developed from embryonic cells of the mesoblast, which swell up and 

 become vacuolated, while their nuclei undergo segmentation. These 

 cells send out protoplasmic processes, which unite with corresponding 

 ones from other cells, and become hollowed, give rise to the capillary 



