Zygnemacece 119 



out a protuberance on the side towards the other filament, and 

 this meets with a similar protuberance from one of the opposite 

 cells. The ends of the protuberances fuse together and an open 

 tube is formed placing the two conjugating cells in communication 

 with each other. This is known as the conjugating -tube. During 

 the development of the protuberances and their ultimate fusion 

 the greater part of the protoplasmic contents of each cell, including 

 the chloroplast, contracts away from the wall of the gametangium 

 and passes into the conjugating-tube. There, a coalescence of the 

 gametes takes place, resulting in the formation of a zygospore, 

 which, although a compact mass situated in the conjugating-tube, 

 is not at first cut off from the rest of the protoplasm left in the 

 gametangia. Wittrock regards the entire H -shaped structure, 

 consisting of the two gametangia and the conjugating-tube, as the 

 zygospore. Cell-walls soon appear in this structure, indicating the 

 formation of a rudimentary sporocarp, and they cut off a central 

 fertile carpospore from the surrounding sterile cells (two, three, or 

 four in number). Thus, the Mesocarpese afford an instance in the 

 green Algae of a sporophyte generation and a rudimentary ' alterna- 

 tion of generations.' 



The method of spore-formation met with in Mougeotia differs 

 materially from that which occurs in the Zygnemeas. The spore is 

 not formed by the development of a new cell- wall around the fused 

 gametes, but by the appearance of partition walls which cut off 

 some part of the H -shaped structure in which fusion of the gametes 

 has taken place. Thus, the spore in the Mesocarpea? can be re- 

 garded as a carpospore which is bounded partly by new walls and 

 partly by the walls of the gametangia or the conjugating-tube. 



The type of conjugation between the cells of distinct filaments 

 is known as scalariform conjugation. In some cases conjugation 

 occurs between adjacent cells of the same filament. This type is 

 known as lateral conjugation, and although commonly met with in 

 the Zygnemea3, is very rarely observed in the Mesocarpese. 



Irregularities are sometimes met with in the conjugation of 

 Mougeotia. Cases have been observed in which the terminal cell 

 of a filament has entered into conjugation through its free end, no 

 conjugating-tube being developed, and rare instances occur in 

 which three cells, each belonging to distinct filaments, have entered 

 into conjugation. Equally rare are the hybrid examples in which 

 conjugation has occurred between species of Mougeotia of different 



