PHYCOMYCETES 



151 



beak-like projections. The developing cushions break the epidermis 

 and the mature spores are set free. Fig. 50, a, shows a section 

 through a conidial cushion. Under favorable conditions germina- 

 tion of the conidia proceeds promptly and each conidium becomes 

 a zoosporangium, the protoplasmic contents dividing into six or 

 more parts which emerge through 

 an opening developed either ba- 

 sally or terminally. The zoospores 

 are set free as ovate swarm cells 

 with two unequal lateral cilia. 

 After a brief motile period they 

 come to rest, become invested 

 with a cell wall, and may push 

 out a germ tube in a few hours. 



The oospores are normally , 

 produced later than the conidia. 

 The oogonia and antheridia de- 

 velop in the intercellular spaces, 

 and the mode of formation is Fig. 51. Fertilization in Cystopus 

 much as in Pythium. The oogonia < After B - M - Davis » 



are, however, in this case larger, measuring from 50 to 60 p in 

 diameter. There are numerous nuclei in the early stages. It is 

 generally agreed that in this species the differentiation of the 

 ooplasm is accompanied by a migration of the nuclei to a pe- 

 ripheral position and the organization of a central body termed 

 a ccenocentrum. A nucleus then returns from this nuclear zone 

 to the region of the ccenocentrum. Preceding the latter, however, 

 it is held that one karyokinetic division may be constantly found. 

 The zone of the now disintegrating nuclei indicates fairly well the 

 line of differentiation between periplasm and ooplasm. As the 

 antheridial tube penetrates, a cell wall begins to be laid down be- 

 tween ooplasm and periplasm. Into the tube of the antheridium 

 a single antheridial nucleus migrates. 



Special attention is called to the fact that at maturity of the 

 egg there is a single nucleus in each gamete, but the egg is also 

 provided with a ccenocentrum. Fertilization proceeds exactly as 

 in Pythium, and during the nuclear fusion the ccenocentrum 

 promptly disappears. Fig. 5 1 shows the oosphere with developing 



