60 - The Cell 



Fig. 3-17. Life cycle of Spirogyro. Conjugation (stage 1) usually occurs in the fall of 

 the year, and during the winter months the zygospore (stage 3) lies dormant. In the 

 spring, however, meiosis (stages 4-7) occurs, and soon thereafter the new haploid fila- 

 ment emerges from the spore capsule (stage 10-11}. 



sure to find and penetrate each egg. The egg agency for sexual reproduction in a large 



cell, on the other hand, is adapted for the majority of plants and animals, there can be 



reception of the sperm and for assembling little doubt as to the efficiency of these devel- 



the maternal and paternal chromosomes to opments. 

 form the zygote nucleus, which will give rise 



to all the nuclei of the offspring. In addition, 

 the egg is laden with yolk and other stored 

 material to nourish the offspring during its 

 embryonic development — that is, until the 

 new organism is capable of obtaining food 

 for itself. And since eggs and sperm are the 



LIFE CYCLES; HAPLOID AND DIPLOID 

 PHASES 



The complete life span of an organism, 

 starting at any given stage, and extending to 

 the time when the offspring reach the same 



GAMETE FORMATION 



^f GAMETE ESCAPING 



GAMETES FUSING 



ZYGOTE MEIOSIS DIVISIONS 



Fig. 3-18. Sexual reproduction of Ulothrix. Each of the four spores may de- 

 velop into a new filament (as shown in Figure 3-14). 



