THE MAINTENANCE OF SPECIES 



XIX 



REPRODUCTION AND LIFE CYCLES 



Preview. Where did life come from ? : Refutation of spontaneous genera- 

 tion ; other theories of the origin of hfe ; Hfe produces Ufe • Regeneration • 

 Asexual types of reproduction : Budding ; fission • Sexual reproduction in 

 the invertebrates : jjrotozoa ; (^tlier invertebrates ; hermaphroditism • Par- 

 thenogenesis • Paedogenesis • Alternation of generations • Sexual reproduc- 

 tion and development in the vertebrates : Germ cells versus soma cells ; 

 fertilization, results of fertilization ; early cleavage and variations caused 

 by yolk ; blastulation ; gastrulation ; mesoderm formation ; early differ- 

 entiation of the embr\'o • Tissue fonnation • Protective devices for the em- 

 bryo : Egg shells ; the yolk sac ; amnion and chorion ; allantois ; placenta • 

 Elaboration of germ cells or gametogenesis : Formation of sperm — spermato- 

 genesis ; formation of ova — oogenesis • The new embryology : Genes ; 

 environment ; natural potencies ; organizers • Suggested readings. 



PREVIEW 



It was obvious to the early philosophers that the earth preceded the 

 living things upon it and they advanced the interesting idea that 

 living things arose spontaneously from their surroundings. The 

 Bible alludes to this belief when Samson propounded his riddle, 

 "Out of the eater came forth meat and out of the strong came forth 

 sweetne.ss." Samson saw flies coming out of the decaying body of a 

 lion, took the flies for bees, which he believed were arising spontane- 

 ously from the lion's body, hence the riddle. The story of the long 

 struggle to disprove spontaneous generation, ending with the conclu- 

 sive demonstrations of Louis Pasteur, makes one of the fascinating 

 bits of reading in the field of biology. 



With the disproval of the existence of spontaneous generation and 

 the perfection of the microscope, great interest was evidenced in the 

 many different ways in which plants and animals reproduced. Today 

 the student of embryology sees the apparently many diverse ways of 

 reproducing the species reduced to a few essentially similar funda- 

 mental patterns. 



Likewise the exactness with which the chromatin is segregated and 

 divided within the developing germ cell is a never-ending source of 



405 



