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 bi nucleate condition 

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



\, I 



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 gonoraeres ; D, two mdi- 

 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 Ascans megalocephala (Boveri, 



1899, 1904, 1910). 



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



