LATER FORMS OF GASTRULA. 201 



of the Guinea-pig (1852), and of the Roe-deer (1854). But it 

 was only quite recently that Eduard van Beneden, an emi- 

 nent Belgian zoologist, was able, owing to the elaborated 

 methods of preparation of the present day, to throw full 

 light on the obscurity which surrounded the germination of 

 Vertebrates, and to give a right explanation of its details. 

 It still, however, remains so difficult to understand these 

 details, that it is desirable to glance first at the germination 

 of Amphibia. In common with Mammals, these animals 

 exhibit unequal cleavage, and form a Hood-gastrula. But 

 the details of germination are simpler and more evident in 

 Amphibia than in Mammals, and they are more nearly akin 

 to the original, palingenetic form of germination. 



The eggs of the common, tailless Amphibia, of the Frog 

 and the Toad, afford the best and most convenient objects 

 for this examination. Masses of them are easily obtainable 

 in the spring from all ponds and pools ; and a careful 

 examination of the eggs with a magnifying glass is suffi- 

 cient to show at least the external features of the egg- 

 cleavage. In order, however, to obtain a correct idea of the 

 intricate details of the whole process, and to understand the 

 formation of the germ-layers and of the gastrula, the egg of 

 the Frog must be carefully hardened, and, the thinnest 

 possible sections having been cut with a razor from the 

 hardened egg, these must be most minutely examined under 

 a ] powerful microscope.^^ 



The eggs of the Frog and of- the Toad are globular in 

 form, and have a diameter of about two millimetres ; tliey 

 are laid in great numbers in masses of jelly, which, in 

 the case of the Frog, form thick lumps, while those of the 

 Toad form long strings. When the opaque, brown, grey. 



