246 



LECTURES ON BIOLOGY 



mantle. Apart from the far greater individual number of the 

 Volvox colony, which is joined together in one organic unit, a 

 fundamental difference does not appear to exist between them 

 and Pandorina. But while there every single body-cell is able 

 to reproduce independently and form a new colony of Pandorina, 

 the cell-individuals of a Volvox colony have lost this old inborn 

 faculty. They are still able to divide, but the parts are no longer 

 able to grow into a young Volvox. Only a few favoured indi- 

 viduals of the cell- state have retained the faculty to preserve the 

 species. We see, therefore, in Volvox a beginning of what we 

 may describe as the essential characteristic of a higher multi- 

 cellular organism : the separation of the whole organism into 

 body-cells and germ-cells (fig. 59). 



If we observe a Volvox colony, 

 slightly magnified we distinguish 

 at once among the numerous cells 

 a few which are differentiated 

 from their neighbours in shape and 

 size, and among these differently 

 constructed cells we discern again 

 two different kinds. They are the 

 rudiments of male and female sex- 

 cells. The microgametes or sperm- 

 atozoa develop first. The semen- 

 globator. forming cells grow rapidly, producing 

 by continual fission within the 

 colony a large heap of a hundred or 

 more spermatozoa about 0'005 milli- 

 metres long. Two long flagelliform processes serve as organs 

 of locomotion. In the meantime the female germ-rudiments 

 (Anlagen] have become more distinct. The absence of flagella3 

 differentiates them from all other members of the colony. 

 Without fission having taken place every female germ, of which 

 the colony contains about twenty to thirty, develops into a large 

 cell of a round shape and rich in plasm, the macrogamete or ovum. 

 When the female and the male sex-cells are mature they separate 

 from the rest of the cells and fertilization takes place in the 

 water. From the fertilized ovum a new Volvox is then gradually 

 developed. 



FIG. 59. Volvox 



SEXUAL HERMAPHRODITIC COLONY 

 WITH OVA AND SPERMATOZOA IN 

 PROCESS OF FORMATION. 



