ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
711 
the circular chromatin body ; at first the disc stains only slightly, but 
with increasing changes of the chromatic substance it becomes darker, 
and has very fine darker grains scattered in it. This chromatic substance 
consists, at first, of a close network, which gradually becomes looser 
and passes into a filamentar coil ; this coil increases in extent, becomes 
looser, and the filaments which form it begin to show cross-structure, 
that is, they consist of spheres set in rows and connected together by 
achromatin. As the cross-structure becomes more and more distinct, 
the particles send off from their surfaces very fine processes into the 
surrounding nuclear substance. The coil disappears as such, and in 
its place “supporting cords,” which are distinctly cross-structured, 
traverse the nucleus ; the processes, meanwhile, continue to increase 
in length ; they, too, show cross-structure, and appear to send off 
processes themselves. From the ends of the ray-like processes the 
smallest cross-pieces are repeatedly broken up, and pass into the nuclear 
substance. In this way the chromatic cords become broken up, and 
finally disappear, while at the same time the nuclear substance becomes 
granular. The grains are so fine that the nuclear substance retains 
almost a homogeneous consistency. 
The essential part in the changes of the chromatic substance of the 
nucleus consist in this, that what is primitively in the form of a close 
network increases in quantity and becomes distributed through the whole 
of the nuclear substance in the form of very fine granules. Finally, 
there are formed six chromatic rods, one end of which is placed close 
to the surface of the nucleus, while the other knob-like end projects 
into its interior ; these rods have probably some relation to the forma- 
tion of directive corpuscles. 
When the work of others is compared with the author’s results, it 
seems clear to him that the nuclear network of Amphibia, Birds, and 
Lizards passes through a regular series of changes during the matura- 
tion of the egg-cell, and there can be no doubt that similar, or very 
similar, changes are to be seen in the nuclei of the ova of other Verte- 
brates. The changes which occur in the body of the cell, in the 
tunica adventitia, and the zona radiata are described in detail, as are also 
the changes which occur in the follicle. 
Development of Pronephros and Segmental Duct in Amphibia.* — 
Mr. H. H. Field has studied the development of these organs in Bana, 
Bufo , and Amblystoma, and he gives a very detailed account of his 
observations. He finds that the first trace of the excretory system is a 
solid proliferation of somatopleure, the pronephric thickening ; the lumen 
of the system arises secondarily, and the pronephric tubules do not 
appear in consequence of the local fusion of the walls of a widely open 
pouch, but are differentiations at an early stage from the hitherto 
indifferent pronephric thickening. 
Taking a wider survey of the Vertebrata generally, the author 
suggests that the pronephros and mesonephros are parts of one ancestral 
organ ; that the glomeruli are strictly homodynamous with the glomus ; 
that the entire tubular portion of the pronephros is represented in the 
mesonephros ; that the cavity of a Malpighian capsule and the nephro- 
* Bull. Mus. Comp. Zool., xxi. (1891) pp. 201-310 (8 pis.). 
