The Beginnings of Multicellular Organization 35 



the male filaments send out huge numbers of gnarled, branched shoots; 

 (2) the female filaments expand at their tips into large sacs; (3) the male 

 shoots grow in a directed manner toward the female sacs and plaster 

 themselves around their surfaces; (4) cytoplasm congeals about the nuclei 

 inside the female sacs, each aggregate of nucleus and cytoplasm being 

 called an oosphere; (5) the male nuclei penetrate the sac through tiny 

 tubes and each unites with a female nucleus to produce a zygote. 



Male and female colonies must be grown together to yield sexual 

 structures. Grown alone, they merely produce the usual filaments with 

 sporangia. Furthermore, sexual structures begin to form before the two 

 colonies actually come into physical contact. That is, they appear when 

 the colony edges are still separated by several millimeters, and develop 

 in a wave, back from the advancing edge of the colonies. Thus, we can 

 conclude that ( 1 ) the presence of a male colony is necessary for the de- 

 velopment of sex structures in a female colony and vice versa, and ( 2 ) the 

 eflPects are mediated by agents, i.e., "sex hormones," that can diffuse 

 through agar over considerable distances. 



In addition, it has been found that each male and female Achlya 

 colony produces several hormones that trigger successive stages in the 

 development of the sexual structures. That is, a hormone A produced by 

 the female vegetative colony induces the male to send out gnarled, 

 branched shoots called antheridiol initials (stage 1 of Fig. 16). Having 

 done so, the male can now produce a hormone B that causes the female 

 vegetative colony to produce the large sacs called oogonial initials (stage 

 2). The female can at this point only produce a third hormone C that 

 causes the male shoots to grow in directed fashion toward the source of 

 this hormone and to plaster themselves around the sacs (stage 3). Upon 

 contact, the male produces a fourth hormone D that causes the female 

 sacs to develop oospheres, and so on. 



Attempts have been made to isolate the first of these hormones ( A ) . 

 Female Achlya was grown in 500 gallons of liquid culture medium, under 

 conditions which permitted the fungus to pour out maximal quantities 

 of hormone A. The medium was collected and concentrated. By exploit- 

 ing various means of chemical purification, the intrepid biologists ended 

 up with 0.0003 grams of material, still impure, but which could act 

 upon the male Achlya when diluted to 1 part in 10,000,000,000,000 parts of 

 water! Unfortunately, 0.0003 grams was too small an amount to permit 

 chemical identification, and this gallant attempt had to be abandoned. 

 Recently, however, the project has been resumed, this time starting with 

 10,000 gallons of Achlya culture! In addition, a similar hormone has re- 

 cently been isolated from another water mold distantly related to Achlya. 

 This hormone, given the romantic name sirenin, has been isolated, purified, 

 and crystallized. Now chemical studies are being done to characterize the 



