GERM-CELL HISTORY IN THE BROOK LAMPREY 69 



which is shown in the section. Numerous cysts are present, 

 some with cells in synapsis, others with degenerating cells, and 

 still others with undifferentiated ceils. 



Figure 74 shows a cross-section of a gland from a larve 65 mm. 

 long. In this larva the gland contains practically only growing 

 oocytes, although a few individual cysts are scattered throughout 

 the gland. Such cysts may contain only actively dividing germ 

 cells or only cells in the various stages of degeneration. Larvae 

 with such glands quite certainly become females. Another gland 

 of this type from a larva 63 mm. long, is shown in figure 75. 



Figures 76 and 77 are two sections from larvae 50 mm. and 60 

 mm. long, respectively. In these the number of oocytes in the 

 gland is greater than the number of cysts. 



Germ cells of the various types mentioned above are distrib- 

 uted throughout the germ glands of this period, apparently 

 without any regularity and without any relation to one another 

 or to the somatic parts of the gland. All of the types may occur 

 along the periphery of the gland or in the deeper portions. Cells 

 may enter the synaptic phases whether they lie singly or in cysts, 

 and irrespective of the position they occupy in the gland. Fur- 

 thermore, no difference has been observed between the follicular 

 cells surrounding those germ cells which have entered the synaptic 

 phases and those enclosing the resting and dividing cells. There 

 is, then, no indication that the somatic environment has anything 

 to do with the initiation of the synaptic phases. The fact, how- 

 ever, that the cells of large cysts which have entered the synaptic 

 phase usually degenerate indicates that the environment of the 

 cells at this time may determine whether or not they shall form 

 growing oocytes. 



In none of the germ glands do all the cells that are destined to 

 form growing oocytes enter the growth phase simultaneously, and 

 therefore, in the same gland one may find oocytes of all sizes, as 

 well as cells in the various stages of synapsis. Figure 78 is a 

 section of the germ gland of a larva 72.5 mm. long, in which are 

 oocytes of various sizes. There is no special hmited period during 

 the course of early development when the germ cells show a 

 greater tendency than at other times to change into oocytes. 



