GAMETOGENESIS AND SPOROGENESIS 



201 



divisions provides them with one nucleus each (Fig, 164). Development 

 of spores by free cell formation occurs in the ascomycetes. After the 

 primary ascus nucleus has divided to eight, a portion of the ascus cyto- 

 plasm is cut out about each of these nuclei, the astral rays curving around 

 from the centrosome having some part in the process (p. 158). Finally, 

 resting spores are often formed as a result of a sexual fusion. Examples 

 of this are the zygospore of Mucor and the oospore of Albugo. 



Bryophytes. — In bryophytes, which show a well-marked alternation 

 of generations, the eggs and spermatozoids are borne by the gametophyte 

 in archegonia and antheridia, respectively. The egg 

 is the innermost cell of the axial row in the arche- 

 gonium (Fig. 118). Commonly it enlarges, rounds 

 up, and lies more or less free in the slimy fluid of the 

 venter, the other axial cells meanwhile breaking 

 down. In some genera (Anthoceros, Sphagnum) the 

 ventral canal cell also is sometimes organized as a 

 functional egg, and there are other facts which sug- 

 gest that the other cells of the axial row were gametes 

 historically. Besides a large nucleus, the egg may 

 contain plastid primordia and chrondriosomes. In 

 mosses Motte (1928) reports that the egg contains 

 practically no food reserves. In many cases a fairly 

 distinct hyaline receptive spot is present at the ^j^ ^^g Arc he- 

 side of the egg directed toward the neck of the gonium of RebouHa, 



nrrhpp-nniiim showing egg, ventral 



arcnegonmm. ^.^^^^i ^^^j ^^^ ^^^j. 



Spermatogenesis has been studied in both liver- neck canal cells. 

 worts and mosses.^ In Marchantia, according to the '^^ '^^^ ' 

 early account of Ikeno, a minute centrosome comes out of the nucleus 

 before each spermatogenous cell-division in the antheridium and divides 

 into two; these pass to opposite sides of the cell and occupy the spindle 

 poles during mitosis. After the last (diagonal) cell-division the centro- 

 some passes to one corner of the cell (spermatid) and there elongates 

 to form the blepharoplast, from which two long cilia grow out (Fig. 119). 

 Meanwhile the nucleus becomes altered in shape and forms the main por- 

 tion of the body of the spermatozoid. In the cytoplasm of the spermatid 

 was seen another chromatic body of uncertain fate. Later accounts of 

 other liverworts present essentially the same story except that the nuclear 

 origin of the centrosome, which is usually seen only in the last mitosis, 

 has not been substantiated. In Blasia the blepharoplast divides into 



6 On liverworts: Ikeno (1903), Miyake (19055), C. E. Lewis (1906), Bolleter (1905), 

 Escoyez (19076), Schaffner (1908), Woodburn (1911, 1913), Sharp (19205), Motte 

 (1928), Yazawa (1931). On mosses: M. Wilson (1911), Woodburn (1915), C. E. Allen 

 (1912, 1917a), Bowen (1926/, 19276), Motte (1928), Weier (1930, 19316). See 

 Miihldorf (1930, 1931) on the mature spermatozoid. 



