RANA. 353 
inner side of the first digit is a rudimentary sixth digit, the 
prehallux. 
Development.—The eggs of the frog are small black spheres, 
about 75-inch in diameter, shed into the abdominal cavity from the 
ovary. They pass forwards into the oviducts and thence to the exterior. 
As they pass down the oviduct they are enveloped in a glassy albu- 
minous matter, which, after deposition, swells up by absorption of water 
into a firm jelly, serving to protect'the egg. 
The eggs are fertilised outside the body by the sperms of the male 
shed over them. The early part of the development is embryonic 
(usually about the first fortnight). During this time the nutrition is 
lecithal (yolk). The later part is larval and the nutrition is herbivor- 
ous. The larvee are termed tadpoles and show striking resemblances 
to fish in their general organisation. After about two months of this 
larval existence a metamorphosis occurs. Great changes in most of the 
organs result in the production of the young frog and the assumption 
of a terrestrial carnivorous existence. 
The Embryonic Period.—The egg is telolecithal, z.¢., the yolk 
is aggregated towards one half of the egg, which is lighter in colour, 
the other half being covered with black pigment. The first two divisions 
of segmentation are parallel to the axis of symmetry, and hence divide 
the egg into equal quadrants, but the third divides it into unequal halves, 
producing four large and four small octants. In further segmentation 
the cells in the pigmented half are produced more rapidly and are 
smaller than those in the yolk-half. 
Hence the segmentation is total but unequal, producing a modified 
blastuda, in which one half has. few-large hypoblast cells and the other 
has many small epiblast cells. Such a blastula is converted into a 
didermic embryo or modified gastrula, not by archiblastic invagination, 
but by egzboly or gradual extension of the epiblast over the hypoblast. 
This overgrowth is not effécted all round the edge of the epiblastic 
portion ; but at one spot, the future hind-end of the embryo, there is 
formed a slight split between the two layers, extending into the hypo- 
blast as the commencing archenteron. This spot, the embryonic rim, 
evidently represents the dorsal edge of the blastopore in Amphioxus. 
Elsewhere, especially at the opposite side, the pigmented epiblast is 
seen to slowly envelop the hypoblast, till eventually there only remains 
a small hole just below the embryonic rim which we may recognise as 
the dlastopore. The epiblast cells are said not to actually grow over the 
hypoblast, but to be continually increased in number and extent by 
actual conversion of the light hypoblast cells into small pigmented 
epiblast cells. The final result is the same, z.¢., that the whole egg 
becomes dzdermic (or diploblastic), an outer layer of epiblast enveloping 
an inner of hypoblast. The small blastopore eventually closes into a 
small longitudinal slit called the sremztzve groove. Meanwhile the 
archenteric cavity has extended inwards by a splitting of the hypoblast 
cells, and the blastoccele or segmentation cavity disappears in front of it 
at the anterior end. 
M. 24 
