GERM-CELL HISTORY IN THE BROOK LAMPREY 55 



cells, the chromosomes are associated in pairs. The paired con- 

 dition persists throughout the various phases of cell division. 

 An association of maternal and paternal chromosomes is appar- 

 ently effected during early cleavages, and probably before the 

 first cleavage, thus continuing from the fertihzed egg to the adult 

 stage. In Diptera the pairing is side by side (parasyndesis) 

 and similar to synaptic pairing. The pairing, according to 

 Metz, comes about through a physicochemical similarity of the 

 homologous maternal and paternal chromosomes. 



In the lamprey the granules on the achromatic network of the 

 resting nucleus form the centers for the reconstruction of the 

 chromosomes. There is little doubt that these granules actually 

 represent chromosomes in all the various phases through which 

 the cells pass; for this reason one may speak of a visible continuity 

 of chromosomes from one cell generation to the next. Thus the 

 individual chromosomes in the germ cells of the lamprey never 

 lose their identity during mitosis. 



The origin, function, and fate of the plasmosomes remain 

 obscure in the cells of the lamprey. In the resting cell there 

 are usually two plasmosomes which appear approximately of 

 the same size and which he some distance apart. As the phe- 

 nomena of prophase advance, they approach each other, and 

 somewhat later only one is present. Whether a fusion of the 

 two takes place or one dissolves and disappears at this time is 

 not known. During late prophase the single plasmosome also 

 disappears or at least is no longer distinguishable among the 

 chromosomes. During the telophase a new plasmosome soon 

 appears in each daughter cell and, as the growth of the cell 

 progresses, a second plasmosome also appears. It is likely that 

 the plasmosomes dissolve during cell division to be formed de 

 novo in the daughter cells. 



b. Synapsis phase of the oocytes. After an indefinite number 

 of divisions, some of the primordial germ cells (oogonia), which 

 lie singly or in cysts, undergo a series of changes preUminary 

 to the stage of actual growth. When the cells have entered this 

 stage they are termed oocytes of the first order (table 1). 



