INVBRTEBRATA. 



403 



8. Reproduction in Protozoa and Metazoa. It has 

 often been said that Protozoa are potentially immortal, 

 while Metazoa must inevitably die. That is to say that in 

 the former death is always the result of some external 

 agent and may be indefinitely avoided, each generation 

 becoming the new one, instead of being replaced by it ; while 

 in the latter old age and death inevitably follow, sooner or 

 later, after reproduction is effected. This distinction is true 

 as long as we consider only such Protozoa as Amoeba, which 

 appears to be capable of indefinite reproduction by fission. 

 But in the case of Paramecium the matter is rather different. 

 The material of its meganucleus is as inevitably doomed to 

 death as the material of the nuclei of ordinary tissue- cells 

 in a Metazoon ; while the immortality of its micronuclear 

 material is, like that of the nuclear material of the ovum 

 and spermatozoon of Metazoa, entirely dependent on ad- 

 mixture with the similar material from another cell. 



In fact, the analogy between the conjugation of Para- 

 mecium and the sexual reproduction of a Metazoon is very 

 close, as the following comparison will show : 



PARAMECIUM. 



A single " rejuvenated " in- 

 dividual divides repeatedly, 

 forming a large number of cells 

 which are all alike, and each 

 of which is an independent 

 individual. 



Each of these cells contains 

 two kinds of nuclear material 

 one (meyanucleus) destined 

 eventually to die ; the other 

 (micronucleus) having a potential 

 immortality, which is, however, 

 dependent on an interchange of 

 part of its material for the 

 similar material of another in- 

 dividual not derived from the 

 same rejuvenated ancestor. 



This process (conjugation) 

 results in two rejuvenated 

 individuals. 



A METAZOON. 



A fertilized ovum divides 

 repeatedly, forming a large 

 number of cells which become 

 differentiated, and are mutually 

 dependent parts of a single 

 multicellular individual [or, if 

 asexual reproduction* also occurs, 

 of several such individuals]. 



These cells are of two kinds 

 one (somatic cells') destined 

 eventually to die; the other 

 (reproductive cells') having a 

 potential immortality, which is, 

 however, dependent on a removal 

 of part of the nuclear material 

 and its replacement by similar 

 material from another repro- 

 ductive cell not derived from the 

 same individual. 



These processes (maturation 

 and fertilization) result in a 

 single fertilized ovum. 



* This will be explained in the next chapter. 



