Lecture X. 8 1 



while the other remains blunt. Two cilia develop from the tapered 

 end. The movements of these cilia at first move the plate as a 

 whole ; later on they shake the cells free from one another and the 

 latter now swim about in the wall-cavity as elongated separate cells 

 the sperms. Finally they emerge from their cavity and swim 

 away to an ovum-carrying individual and penetrating through the 

 wall or entering by the pore may each fuse with and fertilise an 

 ovum. Sometimes the plate of immature cells does not disinte- 

 grate into separate cells until it has penetrated into the female 

 individual carrying the ova. 



The sperms are very small (about io/x x 3/x), pear-shaped 

 masses of protoplasm. Each has a nucleus, a yellow-green 

 chromatophore with pyrenoid, a stigma, and two contractile 

 vacuoles. The two cilia are borne on their tapering ends. The 

 ova are much larger, being about 50^ in diameter. They have a 

 large nucleus, and their dark green chloroplast has many pyre- 

 noids. 



The zygote or cell formed by the union of ovum and sperm 

 develops round itself a hard cell-wall furnished with warty 

 prominences. After a period of rest which coincides with the 

 duration of winter, the outer coat is dispensed with and the 

 contents develop, in the same way as one of the spores, into a 

 new Volvox. 



When we reflect on the structure of Volvox, how every cell is 

 intimately bound together, being in fact just a portion of one 

 coherent mass of protoplasm, with a community of nutrition and 

 other physiological processes throughout, how stimuli and co- 

 ordinating messages are sent from one part of the organism 

 to another, and how the organism responds to these stimuli as a 

 whole, the conviction establishes itself that in Volvox we have to 

 do with an individual. This conviction is rendered more certain 

 when observation shows that Volvox reproduces itself as a whole. 

 In its life history special reproductive cells are set apart, and from 

 these, both new reproductive cells and those which are not 

 concerned with reproduction namely somatic cells are formed. 

 Hence we have good reason to regard Volvox as a multicellular 

 individual. 



The salient feature to which we must now direct our attention 

 is the fact that the organism is differentiated into reproductive 

 and non-reproductive cells (germ cells and somatic cells 

 respectively). It is only the protoplasm of the reproductive cells 

 which continues to live from one generation to another. They 

 form the immortal part of the species. From the moment in 



6 



