Development 61 



system in the new environment. What appeared to be an "acquired 

 characteristic" becomes hereditary through the effects of natural 

 selection. This process, known as genetic assimilation, is discussed 

 in Chap. 7. 



LIFE CYCLES 



Cyclic growth is characteristic of all organisms. Yeast cells go 

 through sequences of fusion and fission, including both a haplophase 

 and a diplophase (Chap. 3). In most higher plants, development 

 occurs both in the haplophase and in the diplophase, although the 

 haplophase (male and female gametophytes ) is usually much less 

 conspicuous and of shorter duration than the diplophase ( the sporo- 

 phyte). In most animals there is virtually no development in the 

 haplophase, which usually is restricted to the gametes. (Male hy- 

 menopterans, which are haploid, constitute a conspicuous exception; 

 see Chap. 9. ) 



The simple growth-fragmentation-growth cycle hvpothesized as 

 the most primitive form of development has been altered by selection 

 in diverse ways. As an example of a complex developmental svstem, 

 consider the protozoans that cause malaria. The sporozoites of 

 Plasmodium, which in a mosquito environment migrate to the sali- 

 vary glands, will, when injected into the blood stream of Homo 

 sapiens, invade specific cell types. Here they may reproduce asex- 

 ually producing merozoites which infect other cells or invade the 

 erythrocytes. Those in the erythrocytes may reproduce asexually, 

 producing merozoites which will infect other erythrocytes, or they 

 may develop into gametocytes and eventually produce gametes 

 which will fuse in the gut of another mosquito. The motile zygote 

 thus formed migrates to the gut wall, and develops into a sporocyst. 

 Sporozoites are formed in the sporocyst by cell division. 



The same genotype responsible for the efficient feeding machine 

 that we call a caterpillar also contains the information needed for 

 the manufacture of the highly dissimilar reproducing-dispersing 

 machine called a butterfly. The zygote that develops into a giant 

 sequoia also contains the information necessary for the manufacture 

 of its tiny pollen grain ( few-celled male gametophyte ) . The single 

 cell of the human zygote, through division, gives rise eventually to 

 such diverse descendants as erythrocytes, muscle cells, and ner\e 

 cells. These deviations from the simplest cycle of development ha\e 

 been in response to selection operating on the entire life cycle of the 

 organism from zvgote formation till death. 



