THE 



POPULAR SCIENCE 



MONTHLY. 



JUNE 1905 



VON BAEK AND THE EISE OF EMBRYOLOGY. 



By Professor WILLIAM A. LOCY, 



NORTHWESTERN UNIVERSITY. 



HPHE process of building an animal's body is one of the most won- 

 J- derful in all nature. After three weeks' incubation of the hen's 

 egg, for illustration, the young bird steps into the world with heart, 

 lungs, brain, eyes and other organs completely formed, which straight- 

 way adjust themselves to the new conditions of life. At the beginning 

 of incubation, this living, breathing organism was merely a single 

 element of structure — a cell, or at most a group of a few cells sur- 

 rounded by a quantity of nourishment in the form of yolk — potentially 

 an animal, in reality simply an egg. 



To arrive at the period of hatching, a succession of changes has 

 taken place whereby the food material has been transformed into the 

 living matter of organic units, and these have become aggregated into 

 the tissues of the body. That such a sweeping change has been 

 wrought in such a short time is a marvel of organic architecture in- 

 volving much more than mere rearrangement of material. 



The history of the development of a single individual becomes en- 

 dowed with greater interest, when observation teaches us that all 

 animals, in the process of becoming, pass through a similar series 

 of steps. In whatever group of living forms the penetrating insight 

 of the scientific observer has been turned, fortified by the micro- 

 scope, there is the same remarkable story — complex living forms arising 

 from relative simplicity to great complexity in a short time. Every 

 organism, above the very lowest, no matter how complex, starts its 

 existence in the condition of a single microscopic cell, and between 

 that simple state and the fully formed condition every gradation of 

 structure is exhibited. Each time an animal is developed these con- 

 structive changes are repeated in orderly sequence. 



vol. lxvii. — /. 



