266 FROaS AND TADPOLES. 



The significance of this process is best understood from a par- 

 allel case in the Lancelot {Amphioxus). In the blastula of this 

 animal, the lower cells are similarly larger, they become flattened 

 and then arch upwards, and finally come into contact with the 

 upper cells; at the same time the lower opening closes, all but 

 a narrow pore. In the frog, on account of the great preponder- 

 ance of yolk cells, this process is not so simple ; the cells on the 

 upper side grow over the lower ones, and the central cavity 

 commences to grow inwards from the point which was left un- 

 covered — the llastopore. This cavity is the future alimentary 

 canal. 



The third stage is now reached, — the gastrula — which may be 

 described as a sac, open at one end. In this condition also, many 

 animals persist throughout their entire existence, such as Sea- 

 anemones and Sydra, etc., which belong to a group known as 

 Coslenterata, which have only one body-cavity, — merely sacs open 

 at the mouth, round which a number of tentacles are arranged. 



The next development is the elongation of the embryo, so that 

 it becomes ovoid. The changes we have just described will take 

 about a week, but in about three to four days more, slight con- 

 strictions are formed, which indicate the division into head, 

 body and tail, though as yet none of these organs are formed. 

 The changes which now set in are very important and compli- 

 cated. The ovoid embryo elongates considerably, the brain and 

 spinal cord are formed along the dorsal surface, and just beneath 

 these the backbone takes its origin. A number of parallel ridges 

 or folds next appear on each side of the head — these are known 

 as the visceral arches, and are six in number. The last four of 

 these are the branchial arches ; and from the first, second, and 

 later the third of these branchial arches, the skin grows out into 

 tufts, forming the external gills. During this time the whole 

 embryo has begun to assume a fish-like form, the tail begins to 

 grow rapidly, the external gills are fully developed and the 

 tadpole is now ready to emerge. It is set free from the gela- 

 tinous mass in about a fortnight from the time that the eggs 

 were laid— it has eyes, but as yet no mouth, it breathes by 

 means of the external gills and swims with its long tail, in the 



