2l8 



ALG^ 



duction in the conjugation of equivalent swarm-cells. Ulva produces 

 two kinds of swarm-spore — megazoospores with four cilia, and micro- 



^vu 



Fig. ig6, — Ulva. a, portion of frond showing cells which produce the swarm-spores ; bj portion of 

 frond with empty cells ; c, megazoospores (magnified). 



zoospores or zoogametes with two cilia ; and 

 these two kinds are produced either on the same 

 or on different individuals. The megazoo- 

 spores germinate directly. Conjugation of the 

 smaller swarm- cells has been observed in Ulva, 

 Enteromorpha, and Monostroma, but the 

 coalescence takes place only gradually ; some 

 time after conjugation four ciha and two 

 pigment-spots are still to be detected. In 

 Ulva the zygosperm thus formed divides re- 

 peatedly after becoming attached to some solid 

 substance ; the cells thus formed separate, and 

 a small colony of multicellular individuals is 

 produced, each of which develops into a new 

 ' frond,' dividing first into a filament and then 

 into a plate. In Monostroma a non-sexual 

 mode of propagation has been observed. At 



Fig. 197. — Entercnnorpha intestinalis FiG. 198.— Stages in the conjugation of the zoogametes 

 Lk. (natural size). (After Hauck.) oi Monostroma buUosnmT\YW. (magnified). 



