REPRODUCTION AND LIFE CYCLES 



415 



an exhaustive series of divisions, whereby each cell repeatedly becomes 

 two (growth). The result of these successive divisions is an enormous 

 multiplication of differentiating cells to form the entire body of the 

 individual (development) . 



In the present connection it is only desirable to emphasize that 

 this complicated process of cell-division (mitosis) has been exhaus- 

 tively studied, so that its essentials are now well known. In a word, 

 the end result is the final distribution, to every one of the innumerable 

 cells that form the individual, of equal germinal contributions from 

 the two parents in the form of gene-bearing chromosomes. 



azrosons. 



heocLJJ 



centrosonze 

 middle 

 piece- 



tail. 



sheccth- 



axial — 

 filament 



>|^^ animal pole 



nacleus 



encL — J 

 pieces I 



vacuole 



vitelline, 



rnembroLne- 



^i-cxnixle 



veg'eLal polt 



Generalized diagram of spernialozoan (left) and ovum (right) ready for 

 fertilization. Note the two views of the spernialozoan. The head contains 

 much nuclear material plus the acrosome. The middle piece contains two disk- 

 like centrosomes, twisted milochondria and cytoplasm, while the tail has an outer 

 sheath and axial filament. Eggs are always larger than spermatozoa and con- 

 tain varying amounts of reserve food. Yolk settles toward the vegetal pole. 

 (After McEwen.) 



Sexual reproduction in the vertebrates is essentially identical 

 regardless of the group considered. In every case there is a special 

 organ in the male called a testis, or spermary, for the production of 

 sperm, and an ovary in the female in which eggs are elaborated. Each 

 sperm or ovum is a single cell. Both kinds of germ cells differ in 

 shape and size throughout the vertebrate series. 



The tadpole-shaped spermatozoa are always much smaller, quite 

 active, and lack nutrient material within their bodies, as contrasted 

 with the sedentary ova in which food is stored for the prospective 

 embryo. Sperm may be divided morphologically into three parts, 



