94 bulletin: museum of comparative zoology. 



process has been carried to an extreme. This form of tetrad is, hke 

 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. Ob\dously, 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 ape 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 i (Plate 4, Fig. (30) 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 "vyell developed 

 asters are seen in the cytoplasm close to the nuclear membrane (Fig. 



