THE ADVANCE OF SCIENCE 



103 



Mivart-Balfour theory to the effect that they are derived 

 from a pair of continuous lateral fins, in fish-like 

 ancestors, similar in every way to the continuous median 

 dorsal fin of fishes. 



The discovery of the formation of true spermatozoa 

 by simple unicellular animals of the group Protozoa is 

 a startling thing, for it had always been supposed that 

 these peculiar reproductive elements were only formed 

 by multicellular organisms (figs. 21, 22, and 23). They 



FIG. 23. 



Spermatozoa (often called " microgametes ") of the unicellular parasite 

 Echinospora found in the gut of the small Centipede Lithobius mutabilis. 



have been discovered in some of the gregarina-like 

 animalcules, the Coccidia, and also in the blood- 

 parasites. 



Among plants one of the most important discoveries 

 relates to these same reproductive elements, the sper- 

 matozoa, which by botanists are called antherozoids. 

 A great difference between the whole higher series of 

 plants, the flowering plants or phanerogams, and the 



