﻿1903] OOGENESIS IN SAPROLEGNIA I'M 



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ruin. Form resemblance must be in complete harmony with 

 cytological conditions to have weight. 



Trow (190 1) has criticised a developmental line that the 

 writer indicated in 1900, which, he states, is an attempt to derive 

 Oomycetes from a zygomycetous ancestry, and which he considers 

 an example of "subjective phylogeny." I have carefully exam- 

 ined what was written in that paper (Davis, 1900, pp. 304-9), 

 and, not finding any reference to specific phylogenetic ancestry, 

 am compelled to suggest to Trow a more careful reading and 

 citation of that article. I presented there suggestions for the 

 developmental history of the sexual conditions in the Perono- 

 sporales from coenogametes derived from the gametangia of 

 algae. These coenogametes at a certain stage in the process of 

 sexual differentiation would be similar to the sexual organs of 

 the Mucorales. The molds were used to illustrate a well-defined 

 sexual condition, which is not at all suggesting that they are the 

 ancestors of the Peronosporales (Oomycetes). 



But the present investigation of Saprolegnia, together with 

 Stevens's (1901) later studies on Albugo, have strengthened my 

 faith in the suggestions of that former paper (Davis, 1900). 

 The Mucorales, Saprolegniales, and Peronosporales are generally 

 acknowledged to be closely related groups, but it seems probable 

 that the affinities are only through the somewhat similar condi- 

 tions of sexual organs derived from the coenogametes of some 

 common ancestry. There are many peculiarities of life-habits, 

 life-histories, and methods of asexual reproduction. Of these 

 three groups the Mucorales present the simplest conditions of 



sexuality and illustrate most nearly the structure of the primitive 

 coenogamete. The Peronosporales and Saprolegniales are dififi- 

 cult to relate to one another, for the higher development of the 

 coenogamete is apparently progressing along divergent lines. In 

 the Peronosporales the protoplasmic differentiation in the 



oogonium determines a centrally placed ^gg in an enveloping 



periplasm, for a single coenocentrum dominates the process of 

 oogenesis. In the Saprolegniales the ooplasm gathers by cleavage 

 around a number of coenocentra, and all the protoplasm passes 

 into the resulting eggs. To the writer the second process seems 



