CHAPTER IV. 



THE DEVELOPMENT OF ANIMAL TISSUE. 



1. The cell or ovum of the female, which is gener- 

 ated in the ovary, passes down the fallopian tube and 

 meets the spermatozoon, the male element, provided 

 conception has occurred. 



2. The ovum there multiplies by indirect division 

 into a mass that resembles a mulberry ; hence the mul- 

 berry stage. 



3. The cells do not all grow with the same rapidity, 

 so that there are soon formed three distinct layers of 



, the original mass, i. e., the cpiblast, the hypoblast and 

 the mesoblast. The epiblast is the uppermost layer, 

 the hypoblast is the under layer, and the mesoblast is 

 the middle layer. 



Now is the time when tissue differentiation begins. 

 All the tissues of the body are formed from these three 

 layers; hence the terms: mesoblastic, hypoblastic and 

 epiblastic, so often used in medical literature. A tis- 

 sue is composed of cells and intercellular substance, 



working in common to accomplish some purpose. 

 3 (33) 



