Ill 



GONOMERY 



79 



nucleus, so that each of the conjugating amoebae has now a single 

 nucleus. These nuclei undergo a process of meiosis, comparable to the 

 formation of the polar bodies of a Metazoan egg, converting each amoeba 

 into a single gamete. The two gamete cells fuse together to form a zygote, 

 their nuclei, however, remaining unfused. Thus the binucleate condition 

 is restored, to be retained through an indefinite number of cell di\asions 



Fig. 36. 



Amoeba diploidea. (After Nagler, A.P.K., igog.) A, the animal in its active phase, showing the double 

 nucleus (gonomeres) ; B, C, division stages showing simultaneous division of the gonomeres ; D, two indi- 

 viduals encysted preparatory to conjugation ; E, in each individual the gonomeres have fused into a single 

 nucleus ; F, conjugation has taken place, and the zygote with the two gamete nuclei (gonomeres) is emerging 

 from the cyst, thus bringing the life cycle back to A again. 



during asexual reproduction, the two gonomeres fusing together for the 

 first time immediately before gamete formation. 



D. THE GERM-TRACK 



One more feature of early development remains to be mentioned. In 

 a large number of animals the primitive germ-cells — those cells, that is to 

 say, that will eventually give rise to gametes — are visibly marked out 

 from the remaining or somatic cells at a very early stage of development. 

 The distinguishing marks may be features either of the nucleus or cyto- 

 plasm. The best-known case is that of Ascaris megalocephala (Boveri, 

 1899, 1904, 1910). 



Nothing remarkable is to be observed in the first cleavage division, 



