1902] STUDIES IN THE FERTILIZATION OF PHYCOMYCETES 421 



phase is reached, however, the nuclei are found arranged in an 

 approximate circle around the region that is to become the 

 oosphere {fig. ^). The arrangement is that of A. Candida rather 

 than of A. Bliti, and is such that no daughter nuclei can re-enter 

 the oosphere region. 



The exodus of nuclei is not complete, otherwise the oosphere 

 would be enucleate and the condition would be like that of 

 A. Bliti. One nucleus and, so far as my observation goes, one 

 only remains behind with the coenocentrum, agreeing in this 

 particular with A, Candida. This nucleus may be seen there later 

 in mitosis {fig. 2). This stage is directly comparable with that 

 of Albugo represented hy fig, 14 in Stevens '01, with the difference 

 that zonation is delayed longer in Sclerospora than in A. Candida, 

 The sequel of this condition is given in fig. j, were one super- 

 numerary nucleus is wandering toward the periplasm, leaving 

 the female pronucleus in contact with or imbedded in the 

 coenocentrum. 



During the completion of the mitosis the ooplasm and peri- 

 plasm become clearly differentiated, but as yet no plasmoderma 

 exists. This process of differentiation has been termed zonation, 

 and is here accomplished by a withdrawal of cytoplasm from the 

 periphery of the oogonium, as in Peronospora (Wager '00, fig. 2) 

 and Albugo Candida (Stevens '01, fig. 14), not by an aggregation 

 of separate dense regions, as in A. Bliti (Stevens '99. figs- 59'^^)' 



At all times during oogenesis there is maintained in the 

 oogonial wall contiguous to the antheridium an unthickened 

 region {figs, i, 4. S)- 



In all the early stages of oogenesis the oosphere is seen to be 

 eccentric, indeed the plasmoderma is usually almost in contact 

 with that side of the oogonium adjacent to the antheridium [fig. 

 4) . In cases of shrinkage it is also observed that the oogonial con- 

 tents adhere firmly to this side of the oogonium, a phenomenon 

 to be observed also in Albugo. 



The coenocentrum is shown in figs. 2-4, and a trace of it still 

 persists xnfig. 5. In Sclerospora this structure is not so definite 

 as in Albugo cajidida and A. Tragopogonis. Its highest differentia- 

 tion is shown in fig. 2, where it consists merely of a region of 



