222 



REPRODUCTION 



the individual.' Examples of this latter type of characters are the 

 color and size differences between the sexes of many birds, the 

 antlers of deer, and even physiological characters like the greater 

 pugnacity of the males in many of the mammals. Thus, the 

 phenomenon of sex, as indicated by sexual differentiation, occurs 

 in many of the simplest forms of animal life and in all many- 

 celled animals, as well as in the great 

 majority of many-celled plants. Some 

 students of unicellular organisms be- 

 lieve that it may exist as a physiolog- 

 ical difference in the absence of visible 

 structural differentiation. In any case, 

 it is so nearly universal that it must 

 have appeared at a very early period 

 in the evolution of organisms. It is, 

 therefore, not surprising that there 

 should be important differences be- 

 tween the sexes in many animals. 



The Germ-cell Cycle. — Historical. 

 — Even in ancient times, the "seeds" 

 of plants and the " germs," or embry- 

 onic stages, of animals vv^ere recognized 

 as the material from which many organ- 

 isms arose, although the general nature 

 of development was a mystery. Eggs 

 of larger size, like those of birds and 

 reptiles, were observed to bring forth 

 young; and the seminal fluid of the 

 male was believed to be necessary for 

 conception in the Mammalia. Ari.stotle 

 (384-322 B.C.) wrote with remarkable 

 acumen upon the problem, considering 

 the facts that were then available. 

 In later times the Italian naturalist, 

 Redi, who showed in 1668 that flies 

 arose from eggs, and not spontaneously, in decaying meat, 

 extended this concept of the egg as the initial stage of development, 

 but such observations did not explain the action of the seminal 

 fluid. After the spermatozoa and the microscopic ova of many 

 animals had been discovered, between 1650 and 1700, the role 



Fig. 111. — Diagram illus- 

 trating the Theory of Ger- 

 minal Continuity. 



A, B, and C represent succes- 

 sive generations; g, gamete (sperm- 

 atozoJn) produced by another 

 organism; z, zygote. White cir- 

 cles indicate successive cell divi- 

 sions within the somatic tissues, 

 the existence of which terminates 

 with the individual of the given 

 generation. Sohd black circles 

 indicate the germ cells. Dotted 

 circles indicate gametes which 

 may perish, or may unite with 

 those of another organism. (From 

 KelUcott, "General Embryology," 

 copyright, 1913, by Henry Holt 

 & Co., reprinted by permission.) 



