44 AN INTRODUCTION TO ZOOLOGY. 



passed down into the muscles ia the course of de- 

 velopment. The distinction between these bones, 

 called membrane bones, and true bones, called 

 cartilage bones, is extremely important to remem- 

 ber, as will be seen when we come to a later chapter 

 (p. 261). But there are very few membrane bones, 

 compared with the number of the true bones. 



The Blood also belongs to the mesoblast layer. 

 Like gristle or skin, it is really a tissue, consisting of 

 similar cells ; only the cells do not stick together, but 

 flow about in a liquid medium. 



The blood of vertebrates contains two kinds of cells, 

 or corpuscles as they are called, the red and the white. 

 The latter come from the fluid lymph (about to be de- 

 scribed), that overflows after receiving the products 

 of digestion ; hence there are always more white cor- 

 puscles to be found in the blood shortly after a good 

 meal. 



Besides the blood, vertebrate animals have another 

 fluid called the Lymph. This is a white fluid with 

 white cells floating in it, instead of red ones, as in the 

 blood. These cells are very like the amceha which 

 we have spoken of as one of the simplest types of 

 the unicellular animal organism, except that they 

 have no contractile vacuole. They are called lymph, 

 corpuscles or leucocytes. They are formed inside 

 certain structures that are known as the lymphatic 

 glands, which are situated in all parts of the body, 

 embedded in the muscles and connective tissue. These 

 glands communicate with each other by pipes, like 



