34 The Beginnings of Multicellular Organization 



The sequence of events is shown in Fig, 17. When small pieces of 

 male and female colonies are innoculated at opposite ends of the Petri 

 dish, they grow toward each other as expanding mats of branched fila- 

 ments (vegetative stage). As they approach each other: (1) the tips of 



Fig. 17. Sexual hormones in /4c/i//o. 



Male and female 

 vegetative filaments 



. Female secretes Hormone A. Male reacts 

 by producing antheridial initiols. 



2. Having reached this stage, male secretes 

 Hormone B. Female is induced to form 

 the oogonial initial. 



3. Female oogonial initial secretes Hormone C. 



This causes the antheridial initials to grow directly 

 toward the oogonial initial and plaster themselves 

 against It. 



4. Male secretes Hormone D. That causes the 

 cytoplasm to congeal about the oogonial nuclei 

 and produce functional female gametes. 



5. Finally, the antheridia, presumably directed by 

 a female hormone, put out germ tubes that 

 connect up with the female gametes and permit 

 entry of the male nuclei. 



