SEXUAL REPRODUCTION 215 
Now the chlamydomonas is an especially in- 
teresting organism inasmuch as it responds 
Fig. 24.—Chlamydomonas media, sexual process. I.—Sexual 
cell (gamete). II.—The same, somewhat more advanced. 
IlI.—The protoplasts, which have escaped from their 
cellulose membranes, just in contact. IV.—The fusion 
nearly complete, note the two nuclei (dark spots), one in 
each gamete. V.—Complete protoplasmic fusion of two 
gametes, but the two nuclei still distinct. VI.—The zygote 
after complete fusion, surrounded by a new membrane. 
with great precision to various changes in 
external conditions which are able to influence 
