262 



SPERMATOGENESIS AND OOGENESIS 



the water. After a time the sphere breaks up, the constituent 

 zooids (c) swimming about independently, and finally losing 

 their cilia and becoming amoeboid (D). The amoebulae, in all 

 probability, become encysted (E), forming egg-like bodies 

 which by repeated division of their protoplasm (F) produce 

 the spherical colony. 



It is obvious that Magosphaera resembles the polyplast 

 stage of an embryo : moreover it is produced by the repeated 

 fission of an amoebula just as the polyplast is formed by the 

 repeated fission of an oosperm. 



FIG. 64. Magosphcera planula. 



A, the entire colony, surface view, showing the numerous ciliated 

 unicellular zooids. 



B, the same in vertical section, showing the zooids imbedded in a 

 gelatinous intermediate substance. 



C, a single free-swimming zooid. 



D, the same after assuming an amoeboid form. 



E, the same encysted. 



F, the encysted form with its protoplasm dividing to form a new 

 colony. (After Haeckel.) 



The beautiful Volvox (Fig, 65), one of the favourite 

 studies of microscopists, is a colony of Haematococcus-like 

 zooids arranged in the form of a hollow sphere. Each cell 

 (c) has a nucleus, a contractile vacuole, a large green chroma- 

 tophore, a small red pigment-spot like that of Euglena (p. 47) 

 and two flagella : by the combined movement of all the 

 flagella a rotating movement is given to the entire colony. 



