THE SPERMATOGENESIS OF LEPIDOSIREN PARADOXA. / 
lobed nucleus. Sometimes appearances strongly suggestive of 
amitosis are observed, but I liave never seen any sign of 
accompanying cell division, and I have little doubt that 
direct division does not occur. In the cleaving egg of 
Lepidosiren the nuclei of the blastomeres are often strongly 
lobed, but when the prophase sets in these lobes are drawn in 
and the nucleus becomes more and more spherical in outline. 
Although I have not seen this occurring in the case of the 
polymorphic spermatogonia (probably because the rounding 
off takes place before the prophasic changes are apparent), it 
seems most probable that this happens also in the case of 
these nuclei. The polymorphic nuclei are always especially 
rich in the diffuse substance already mentioned. The later 
generations of spermatogonia differ from the earlier ones 
chiefly in size, both of nucleus and cell-body. At a certain 
stage in its life-history the spermatogonium seems to pass 
into a period of more rapid division, and it is this rather ill- 
defined point that may be taken as separating the primary 
from the secondary spermatogonia. In consequence of the 
successive rapid divisions the size of the nucleus decreases 
from about 30 g to about 17 /* or less in diameter. Fig. 2 
shows a nucleus belonging to this latter period. Its structure 
is the same as before, but there is none of the darkly staining 
ground substance shown in PI. 1, fig. 1. I. have never found 
one of these smaller nuclei polymorphic. 
Although it should logically come at this point, it will be 
best to delay the description of the spermatogonial mitoses 
till after the meiotic (maturation) divisions have been des- 
cribed, as this will bring out more clearly the significant 
differences between the meiotic and pre-meiotic prophases. 
The last generation of small spermatogonia gives rise to 
the primary spermatocytes, which increase during the 
growth period till they are larger than the largest spermato- 
gonium, being frequently over 30 in diameter at the time 
that they are preparing to enter the meiotic prophase. The 
resting nucleus of the primary spermatocyte at the end of 
the growth period has a very characteristic structure, very 
