GERM-CELL HISTORY IN THE BROOK LAMPREY 59 



or less grouped. There is no evidence that they are derived from 

 the nucleus, so they may be considered as true cytoplasmic 

 bodies. The mitochondria in the early oocytes of the lamprey 

 are not essentially different from those found in the indifferent 

 germ cells (figs. 41 and 52). 



During the progress of the early nuclear changes, up to the 

 time of the synaptene stage, there is a gradual decrease in the 

 amount of cytoplasm and a gradual disappearance of the mito- 

 chondria. After the synaptene stage the amount of cytoplasm 

 increases again, but no study was made of the mitochondria sub- 

 sequent to the synapsis phases. 



In the undifferentiated germ cells there is an astrosphere near 

 the nucleus, even in the resting cells. Sometimes a minute cen- 

 trosome has been distinguished in the middle of the astrosphere. 

 The astrosphere may be distinguished from the surrounding cyto- 

 plasm by its more granular appearance. During the middle 

 synapsis phases it seems to disappear with the decrease in the 

 amount of cytoplasm, but it reappears in the early growth 

 period and remains through this whole period as a very promi- 

 nent structure (fig. 60). 



The vitelline body may be traced through the various phases 

 of the synaptic period, and during the growth period it becomes a 

 very conspicuous structure (figs. 60, 62). 



Discussion. The general history of the period has been out- 

 lined above, but certain features require further discussion. 

 The changes taking place in the cells during this period initiate 

 the period of heterotypic or reduction division. This is a period 

 through which all germ cells apparently must pass before they 

 can become functional ova or spermatozoa. In the female cell 

 these changes take place at a very early stage in the development 

 of the animal, and in the case of the lamprey they precede the 

 maturation period proper by at least two or three years. In the 

 male lamprey these changes occur much later in the life of the 

 individual and usually precede the maturation divisions by only 

 a very short time, probably not more than three or four months. 



I have made a study of the synaptic phases of the male germ 

 cells in the lamprey and found the process of development to be 



