532 



THE DEVELOPMENT OF THE INDIVIDUAL 



growing parts and to remove the wastes. Within 41 hours after the egg 

 of a chicken begins incubation, a functional circulatory system will have 

 developed with blood vessels containing blood corpuscles and a beat- 

 ing heart in the center of the system. If we open an egg which has been 

 incubated about 48 hours we can see the flattened embryo on the upper 

 side of the yolk mass. This embryo can be cut off with a sharp pair of 

 scissors and floated on water in a watch glass. When viewed under a 

 low powered microscope the heart can be seen pumping blood forward 

 to the gill clefts and even the tiny blood corpuscles can be seen pulsing 

 through the vessels. It is also possible to see the capillaries connecting 

 the arteries and the veins. At first the heart is a simple tube, but it 



*7^V, 



Courtesy General Biological Supply House 



Fig. 33.7. A 48-hour chick embryo showing the advanced state of development of 



the blood vessels and the heart. Since food and oxygen are brought in by the blood, 



the circulatory system develops very early in embryology. The vessels have been 



injected to make them show more clearly. 



later becomes divided into an auricle and a ventricle. In the fish the 

 heart remains in this condition, but in the frog a septum divides the 

 auricle into right and left chambers. In the birds and mammals the 

 ventricle as well as the auricle is divided into two chambers. 



The Skeletal System. This system also originates from the meso- 

 derm. The inner portion of each somite develops a region known as 

 the sclerotome which breaks up into a mass of cells that migrate in and 

 around the spinal cord and notochord and there form the vertebrae. 

 Some of these wandering cells, known as mesenchymal tissue, also mi- 

 grate into the developing limb buds and form the bones of the arms 

 and legs. There are two types of bone according to their method of 

 origin. Membrane bones, which are found only in the head region, 



