62 EIGHTEENTH REPORT. 



ently progressed least from the hypothetical ancestral forms) 

 these uniting cells (gametes) are alike so far as the most care- 

 ful scrutiny will reveal. However, as we follow up the various 

 evolutionary lines we find that in most of these isogamy is pre- 

 valent at the base of the line with a gradual transformation to 

 heterogamy toward the apex of each line. It is probably beyond 

 dispute that this change from isogamy to heterogamy has taken 

 place independently in many distinct lines. Thus in the Volvocales 

 some species of Chlamydomonas are isogamous, but Volvox is 

 hetergamous; in the Phaeophyceae some species of Ectocarpus are 

 isogamous and other species of the same genus show stages of 

 heterogamy varying from merely a distinction of sluggish and ac- 

 tively moving, but otherwise indistinguishable gametes to an actual 

 difference in size as well as activity. Higher up in the same great 

 group we find that the larger gamete has lost its motility en- 

 tirely. A similar development is seen in the Chlorophyceae as we 

 pass from isogamous forms like Ulothrix or Stigeoclonium through 

 various stages of heterogamy to forms like Oedogonium and 

 Coleochaete. Even in the Conjugatae we find that Mougeotia is 

 strictly isogamous while in the closely related Spirogyra there is 

 a distinction of sex in that the protoplasm passes out of one cell 

 (male) into another cell (female). Some of the Protozoa are iso- 

 gamous and closely related forms heterogamous. However, in the 

 Animal Kingdom heterogamy entered at a relatively earlier stage 

 of evolution than among plants. 



Let us return to the question as to what are the processes taking 

 place in sexual reproduction. In plants like Ulothrix or Ectocarpus 

 we see the union of two naked, motile cells of equal size. (Fig. 2.) 

 In Fucus we see the union of small, motile almost colorless cells 

 (sperms) with large non-motile, deeply colored cells (eggs). In 

 Spirogyra the protoplasm of one cell crowds through a narrow 

 conjugation tube to unite with the protoplasm of the other cell. 

 In all these cases it is whole cells that unite. If we turn to the 

 fungi we find in Albugo one or many (depending upon the species) 

 male nuclei and probably some cytoplasm passing through a con- 

 jugation tube into the oogone; in Pyronema it is many nuclei and 

 probably some cytoplasm that pass from the antherid into the 

 trichogyne and thence into the oogone. In neither case, however, 

 does all the cytoplasm of the antherid pass over, so that it is ap- 

 l)aront that a union of com])lete cells is not necessary to the process. 

 In the flowering plants the male cells enter the pollen tube as true 

 cells, i. e., nucleus and cytoplasm, but in their passage down through 



