PHYSIOLOGY 191 



excreted by the skin. Most of the water to be excreted is taken 

 from the blood in Malpighian corpuscles. 



Some of the nitrogenous wastes are excreted in the form of am- 

 monium salts and some free or combined amino acids. However, 

 most of the ammonia which results from protein metabolism is con- 

 verted into urea in the liver and is carried in that form to the kid- 

 neys where it is removed from the set of capillaries ramifying over 

 the convoluted tubules by a process of true secretion. According to 

 this idea, the urine which consists of urea, various salts, other soluble 

 materials, and water is excreted by different parts of the uriniferous 

 tubule. The substances which are excreted by the kidney are not 

 formed there, but are merely removed from the blood by this organ. 



Reproduction 



A living organism is in numerous ways similar to a machine, but 

 reproduction of new units of living material by existing organisms 

 is hardly comparable to any mechanical processes known in our 

 industries. New organisms all arise from preexisting organisms of 

 the same kind. The process of cell division is the fundamental basis 

 for all reproduction. For centuries before the invention of the 

 microscope it was commonly believed that living things arose spon- 

 taneously from nonliving material, or from the dead bodies of 

 plants and animals. Certain old books carry directions for the 

 artificial generation of mice or bees. Louis Pasteur did as much as 

 anyone to discredit this idea of spontaneous generation. Our pres- 

 ent conception is that the protoplasmic substance of the new indi- 

 vidual is but a continuation of the specific protoplasm peculiar to 

 an earlier individual or in sexual reproduction to two individuals. 

 Therefore, under ordinary circumstances the structural and physio- 

 logical complexities which arise through embryonic development 

 must be generally similar to those of the predecessors. 



In most of the single-celled organisms reproduction may occur 

 by such equal division of the protoplasm (binary fission) that the 

 new individuals cannot be distinguished as parent and offspring. 

 Protozoa may reproduce also by sporulation, by which process the 

 cell forms a protective cyst and by a series of simple divisions (frag- 

 mentation) the internal protoplasm breaks into a number of smaller 

 units. Following this the cyst ruptures and releases these new units 

 as independent individuals. For the most part, reproduction among 



