210 ORIGIN OF SEX AND OF THE SOMA 



between the two cell cavities. The cell bodies of the 

 thread which began the process (male gametes) now slip 

 through the canals and fuse with the bodies of the 

 opposite cells of the other thread (female gametes), 

 which have meanwhile contracted away from the cell 

 wall, and a zygote is thus formed in the cavity of 

 each female cell (Fig. 31, D). The zygote becomes 

 covered by a thick wall and ultimately germinates to 

 form a new Spirogyra thread by splitting of the thick 

 outer wall and protrusion of thC: inner thin wall 

 (Fig. 32, C). Cf. the germination of the zygote of 

 Mucor (p. 163,'and Fig 16, i). In some species, however, 

 adjacent cells of the same thread may conjugate 

 (Fig. 31, C), one acting as the male the other as the 

 female cell, and in other cases, again, parthenospores may 

 be formed, quite similar to zygotes in appearance and 

 germination, but each produced from the contents of a 

 single cell without conjugation. 



The sexual differentiation of the gametes of 

 Spirogyra is mainly seen in the active and passive 

 roles of the two conjugating cells : it does not involve 

 a difference of structure or even of size. But in pne 

 species at least there is an interesting change in the 

 structure of the male gamete after it becomes part of 

 the zygote, its chloroplast degenerating and disin- 

 tegrating in the zygote (Fig. 32, A, B), so that only the 

 chloroplast derived from the female gamete remains 

 and gives rise to the chloroplasts of the new thread 

 formed on germination. This is a late occurring 

 degeneration of the nutritive equipment of the male 

 gamete, which may be compared with the reduction 

 or disappearance of the chloroplast in the ordinary 

 sperm before the formation of the gamete (cf. p. 200). 

 Other slight indications of " maleness " may be found 



