3i8 



GENERAL PRINCIPLES 



distinguished one from the other. These swarm spores unite in 

 pairs, and from each pair a single protoplasmic mass is formed, 

 which is, in fact, a single cell. Spirogyra is another filamentous 

 alga similar to Mougeota, but when two filaments unite by 

 tubes the protoplasts of one flow completely across to the other, 

 where the union takes place; the protoplasts of the second cell 

 remaining passive. In another example of swarm spores, two 

 kinds of spores are produced. The difference is not great, but 



Fig. 182.— Conjugation and differentiation of sex. A, Conjugation in Mou- 

 geota; B, conjugation in Spirogyra; C, conjugation in Hydrodictyon (isogamous) ; 

 D, conjugation in Chlamydomonas (heterogamous). In C: i, a gamete; 2, con- 

 jugation; 3, zygote. In D: i, a. male gamete; 2, female gamete. 



one is somewhat larger than the other. Here the union takes 

 place between individuals of different kind. In Fucus, a 

 brown sea weed, the two kinds of cells differ greatly in size. 

 The large one is motionless, while the small one has two 

 flagella and is very active. In all the higher plants and 

 animals the difference in size is enormous, and in animals espe- 

 cially, the smaller motile cell consists of little more than 

 a small and compact nucleus with a single flagellum. 



694. Fertilization. — In the lower forms, where the two unit- 

 ing cells are equal in size or nearly so, the process in question is 



