THE VITAL PROPERTIES OF THE CELL 



285 



We may take either Botrydium or Ulothrix (Fig. 155) as an 

 example of isogamous fertilisation. If minute swarm-spores from 

 different sources are placed in a drop of water and examined with 

 a hi^h power of the microscope, some of them are seen to approach 

 e.ach other immediately, their 

 hyaline anterior ends (6) com- 

 ing into contact; and after a 

 short time they commence to 

 fuse together. At first they 

 touch each other laterally (c), 

 after which they grow to- 

 gether, the fusion commencing 

 at their anterior ends and gra- 

 dually extending backwards. 



The couple (d) hurry about 

 for some time in the water 

 with an intermittent and stag- 

 gering movement. After a 

 short time the fusion is so far 

 advanced that the two gametes 

 form a single thick oval body, 

 which, however, betrays its 

 derivation from two indi- 

 viduals by containing two pig- 

 ment spots and four flagella 

 (e,f). The zygote now gradu- 

 ally slackens its movements, 

 until finally it comes to rest ; 

 it then loses its four flagella, 

 which are either drawn in or 



thrown off, becomes globular in shape, and surrounds itself with 

 a cell- wall. 



Frequently the resting stage begins only a few minutes after 

 the commencement of pairing ; in other cases, however, the zygote 

 may swim round in the water with its four flagella for three 

 hours, in a naked condition, without a membrane, until finally it 

 draws in its flagella, and sinks to the ground. 



The gradual appearance of sexual differentiation can be 

 followed still better in the very numerous species of lower Algse, 

 in which the fertilisation of gametes occurs. 



As in Spirogyra (Fig. 153), one of the two individuals, which 



FIG. 155. Botrydium grannlatnm (after 

 Strasbnrger, Fig. 139) : A free plant of 

 medium size (x28); B swarm-spore, fixed 

 with iodine solution (x5K)); C isogametes. 

 a a single individual ; b two isogametes 

 which have just come into contact; c, d, 

 and e the same lying side by side ; / zygote, 

 produced by the complete fusion of the 

 gametes (x510). 



