THE BIOLOGY OF DAILY LIFE. 49 



The young " swell," usually called " embryo " (from 

 the Greek wflpvov = /r6s /tyvoi/, that which swells 

 within*) grows thus : 



In the egg, or ovum (from which every animal 

 begins its life), there is this embryo. It starts in life 

 by dividing into two, and these parts again subdivide 

 indefinitely until a mass like a mulberry is formed. 

 Some of this mulberry-mass forms into a layer called 

 BLASTO-DEKM (Grerm-skin or film). This germinal 

 layer soon differentiates into three layers called (by 

 names meaning "upper," "middle," and "lower") 



1. EPIBLAST. 



2. MESOBLAST. 



3. HYPOBLAST. 

 From which are developed respectively 



1. Skin, Brain, and Nerve centres. 2. Main 

 tissues and organs of body, arteries and veins, 

 muscles, nerve-cords. 3. The epithelium, 

 or lining of digestive and respiratory tracts 

 in a word, the inner skin, or lining of the 

 body. 

 We must now call in Dr. Klein : 



"16. Development of Blood Cor- 

 puscles. At an early stage of embryonic 

 life, when blood makes its appearance, it is a 

 colourless fluid, containing only white cor- 

 puscles (each with a nucleus), which are 

 derived from certain cells of the mesoblast. 



" These white corpuscles change into red 

 ones, which become flattened, and their proto- 



* Not "from within," i.e., its own substance, as some have 

 translated, trying to read modern embryology into ancient Greek. 



E 



