1 70 READINGS IN BIOLOGICAL SCIENCE 



A multicellular embryo begins life as a fertilized egg. The fertilized 

 egg straightway becomes a ball of cells, which soon organizes into three 

 sheets known as the primary germ layers. From these the tissues, organs 

 and body rapidly emerge. At the end of the developmental period the 

 adult body may be many billions of times bulkier and heavier than the 

 original egg. 



It seems like a long span from the egg to the trillions of cells that com- 

 prise the completed body of man, yet this prodigious final number can 

 be attained quite readily by repeated cell division. So rapid is the doubling 

 process that some 45 generations of mitoses are sufficient. 



The first important move toward organization in a young embryo 

 establishes three superimposed, cellular plates, the primary germ layers. 

 From their positions they are termed the ectoderm (outer skin), mesoderm 

 (middle skin) and entoderm (inner skin). Since the ectoderm covers the 

 body it is primarily protective in function, but it also gives origin to the 

 nervous system and sense organs through which sensations are received 

 from the outer world. The entoderm, on the other hand, lines the primi- 

 tive digestive canal and is from the first nutritive; later it also becomes 

 respiratory. The mesoderm, lying between the other two layers and later 

 splitting into two sheets, naturally performs the functions of circulation, 

 muscular movement, excretion and reproduction; it also gives rise to the 

 skeletal structures which support the body. The germ layers are however, 

 not so absolutely specific in their potentiaUties as was once thought. 



HUMAN SEX CELLS 



Although always relatively large, the exact size of a mature ovum is 

 correlated with the amount of stored food substance and not with the size 

 of the animal producing it. The smallest eggs are those of the mouse and 

 the deer (about 0.07 mm.); the largest have a diameter measurable in 

 inches (birds; sharks). Most ova are nearly spherical in form and all pos- 

 sess the usual cell components. There is little difference in the size of the 

 eggs formed by the various placental mammals; mouse, man and whale 

 are nearly equal in this respect. The mammalian egg is small in compari- 

 son with many ova; yet when set beside ordinary cells it is truly big, since 

 it is just visible to the naked eye as a tiny speck. The diameter of normal, 

 fresh specimens of human ova is now known to be about 0.135 "ini. Never- 

 theless, all the eggs necessary to replace the present population of North 

 America could be placed in a cubical vessel three inches square. 



At one time human sperm cells were regarded as parasites, and under 

 this misapprehension the name spermatozoa or 'semen animals,' was given 

 them. Although its length is nearly one-half the diameter of a human ovum, 

 the relative volume is only as 1:85,000. All the spermatozoa required to 

 produce the next generation of North America could be contained in a 

 spherical vessel having the diameter of an ordinary pinhead. 



