DEVELOPMENT OE PAT-BODIES IN RANA TEMPORARIA . 135 
plentiful supply of food-yolk, and is consequently independent 
of nutrition obtained from without. But now, as it grows, the 
absorption of the food-yolk proceeds more rapidly, while at the 
same time certain changes are observed, notably the gradual 
atrophy and subsequent disappearance of the external gills, 
while the recently acquired internal gills take on more work. 
If at this stage the pronephros be examined, it will be seen 
that the tubules have a narrower diameter than those of the 
younger tadpole, while the cells are not so clearly defined. 
By this time the number of funnel-like openings into the 
body-cavity has increased from three to five, and a new and 
important structure has made its appearance, the mesonephros, 
developed, as Sedgwick has shown, 1 in the mesoblast inde- 
pendently of the peritoneal epithelium. 
The meso- and meta-nephros are not distinct from one another 
in the tadpole ; they together form the kidney as found in the 
adult, and it is in this sense that the word kidney will be used. 
The mesonephric tubules extend gradually from behind 
forwards till they come in contact with the pronephros. The 
whole nephros then acquires a distinct capsule, becomes sepa- 
rated from the muscular substance of the lateral mass, and 
lies freely in the abdominal cavity on the ventral aspect of the 
vertebral column, the peritoneum passing over it. Between 
the two kidneys is the aorta (fig. 5). The genital organs, 
which arise as two hollow ridges, also gradually separate from 
the body wall, lying internal and ventral to the kidneys (fig. 5), 
and are still perfectly well defined anteriorly, the proper 
genital substance extending quite to the anterior end. 
Concurrently with these changes of conformation, the 
structure of the pronephros has been undergoing modification 
of the nature of a fatty degeneration. At the time that the 
hind limbs are just making their appearance, the degeneration 
has gone on to the extent represented in fig. 4. 
1 “ On the Early Development of the Anterior Part of the Wolffian Duct 
and Body in the Chick, together with Some Remarks on the Excretory 
System of the Vertebrata,” ‘ Quart. Journ. Micr. Sci./ vol. xxi, N. S., 
1881, p. 449. 
