4 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
process has been carried to an extreme. ‘This form of tetrad is, like 
the former type, derived from two longitudinally split rods, and in 
the same way, except that here the process is carried much farther. 
The halves of each rod diverge at the point of union of the univalent 
autosomes along a line at right angles to the longitudinal split, while 
at the same time the free ends of each pair of arms bend toward each 
_ other. Obviously, in this type, where the two pairs of arms may be 
of nearly equal length, it is often impossible to recognize the point of 
union of the univalent components, whether at the ends of one or the 
other pair of arms. 
‘The third type of tetrad may be looked on as a still further modifi- 
cation of the common fundamental form. In this type the halves of 
the split rods do not diverge along the line of union of the univalent 
components to as great an extent as in the preceding type, so that the 
arms developed along this line are always shorter than the others. By 
the bending of the two longer arms their tips approach each other until 
the arms meet or even cross each other. This gives rise to closed 
rings or crossed loops (Figs. 170-178). ‘The latter when viewed in 
a certain direction may appear x-shaped. ‘The free ends of the crossed 
loops may unite giving rise to a figure of 8. Extending out from the 
tetrads in all directions are fine linin fibrils of varying lengths, which 
give to the tetrads a characteristic hairy appearance. ‘The longer 
fibrils are continuous with a well defined linin network which extends 
throughout the nucleus. 
The later changes in the structures of the tetrads appear to be due 
entirely to a concentration of the chromatin. 
In stage 2 (Plate 4, Fig. 60) the chromatin becomes much more 
compact, while the longitudinal split is much less distinct, and at the 
end of this stage (Fig. 61) has become almost entirely obliterated. 
‘The tetrads for the most part are arranged around the periphery of 
the nucleus in contact with the nuclear wall. As in the preceding 
stage, there are large numbers of linin fibrils extending out from the 
tetrads in all directions. ‘The linin network is more highly developed 
than at any previous period; so much so, in fact, that the nucleus 
often appears nearly as dense as the surrounding cytoplasm. 
‘The mitochondrion is more abundant at this stage than at any other 
time and forms irregular, very finely granular masses which are 
distributed around the nucleus at irregular intervals and stain with 
Bordeaux. 
At this time two minute centrosomes surrounded by well developed 
asters are seen in the cytoplasm close to the nuclear membrane (Fig. 
