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 yV-iiich 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 larvae 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 tKe production of the young frog and the assumption 

 of a terrestrial carnivorous existence. 



The Embryonic Period. — The egg is telolecithal, i.e.^ the yolk 

 is aggregated towards one half of the Qg<g^ 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 

 blastula^ 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 epiholy or gradual extension of the epiblast over the hypoblast. 



This overgrowth is not effected 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 blastopore. 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, i.e.^ that the whole egg 

 becomes didermic (or diploblastic), an outer layer of epiblast enveloping 

 an inner of hypoblast. The small blastopore eventually closes into a 

 small longitudinal slit called the primitive groove. Meanwhile the 

 archenteric cavity has extended inwards by a splitting of the hypoblast 

 cells, and the blastocoele or segmentation cavity disappears in front of it 

 at the anterior end. 



M. 24 



