AN INTRODUCTION TO ZOOLOGY 



latter is sometimes referred to as the body cavity, 

 sometimes as the stomach, the animal being said to 

 possess 110 body cavity. 



Parts derived from the Mesoblast. At a 

 very early stage of the embryo, the mesoblast is 

 formed between the two primary layers (see p. 30). 

 At first its cells are all alike, but afterwards they 

 develop into tissue of different kinds, including the 

 supporting tissues of the body, namely connective 

 tissue and bone, and the muscular tissue. Connective 

 tissue is a thin transparent tissue, which binds to- 

 gether all the other parts. 



Fig. 10.— Typical Mesoblast Cells (sea p. 72) from aii embrvo chick of three 

 days' incubation, ijighly miigrjifled. The large oval nuclei of'lhe ctlls have a 

 round nucleolus, or sometimes two. 



Muscles are of two kinds. The smooth muscles 

 perform the actions that are not governed by the 

 will, and are called reflex actions. They are called the 

 involuntary muscles. 



The other kind, known as the voluntary or striped 

 (striated) muscle, composes the mass of what we usually 

 call "the muscles." These are striped with rings 



