AMBLYSTOMA PUNCTATU3I. 11 



at once seen that there is great variation in the progress of devel- 

 opment. The position of the balancers too, varies considerably 

 in different individuals of the same age. 



Active growth is next shown in the development of the tail 

 and the caudal and dorsal fin ; the branchite and supporters are 

 also growing rapidly, and a depression on the ventral side, on a 

 line between the eyes, marks the position where the mouth will 

 appear. The heart may still be seen in the pericardial region, 

 though the integument is gradually becoming more opaque. It is 

 now making from forty-eight to fifty pulsations per minute. Plate 

 8, Figure 16. During the following thirty -six hours, the branchise 

 continue to progress rapidly, becoming more and more elongated, 

 and begin now to bud out small processes from the sides. The 

 eye has become much more perfect, and its structure is nearly com- 

 plete. The balancers have grown with the gills, though they do 

 not equal the latter in length. The caudal fin has become so 

 large that it now performs its functions as the locomotor organ of 

 the body. The animal shows quite active energetic movements in 

 the egg, and if it is allowed to escape into the water by tearing 

 open the membranous shell, it is seen to swim about with great 

 activity, being j)ropelIed by vigorous movements of its tail. 

 Watching its movements as it sinks to the bottom of the dish, 

 which is covered with a deposit of fine, light, vegetable debris, we 

 can readily determine the use of the balancers. As the animal 

 approaches the bottom it holds its balancers out from the body so 

 that they point outwards and downwards; owing to this position 

 in which they are held, the animal sinks but a short distance into 

 the light material of the bottom and thus keeps the head and 

 branchise above the dirt where they can be readily furnished with 

 a constant supply of pure water. The pericardial region is at the 

 same- time kept free from the bottom, so that there is nothing to 

 interfere with the beating of the heart. Plate 3, Figure 17. This 

 arrangement calls to mind the position which the cuttle-fish, 

 Loligo, assumes when at rest; the tail and posterior portion of the 

 body rest directly upon the bottom while the anterior portion is 

 supported entirely by the median ventral pair of arms, only the 

 anterior or distal ends of which furnish a support for the anterior 

 portion of the body; the rest of the arms are arched so that the 

 head and neck are kept from touching the bottom ; thus affording 

 free opportunity for the egress and ingress of water to and from 

 the mantle-cavity and free use of the siphon. 



