12 S. F. CLARKE. 



A ventral view at this stage shows that the pericardial region is 

 moved slightly further l)aek, the neck region is not so narrow and 

 the neck groove is continuous across the ventral surface. The 

 outline of the mouth is indicated ; the gill processes are increasing 

 in size and in number; the balancers are still growing and have 

 become somewhat capitate and the brachial lobes are beginning to 

 increase in size. The head too is now changing shape, becoming 

 much broader. 



It is interesting and suggestive to note in a ventral view at this 

 period, the general resemblance to a young dog-fish, especially in 

 the position of the mouth and branchis and the shape of the head 

 and body. 



For the next two or three days development is most active in 

 the branchite and in the tail. The latter increases considerably in 

 length and the dorsal fin grows rapidly. The branchiae double 

 their length in two or three days and give off numerous processes 

 which grow rapidly and which are arranged in two rows, the 

 members of which point outwards and downwards, diverging from 

 each other. The brachial lobes are develo]iing slowly, being as 

 yet, a pair of simple lobes or processes on the sides of the body just 

 behind the branchiae and partly covered by the latter. The change 

 in the form of the head continues; it is becoming more rounded 

 in front and broader. From this time until the posterior pair of 

 limbs are being developed there is very little change externally, in 

 the posterior portion of the body. The branchiae and supporters 

 have now reached their full development; that is, tlie branchiae 

 have all their processes budded out and the branchiae are relatively 

 to the size of the body as large as they ever will be, though 

 absolutely they will still increase in size; the balancers, however, 

 being only embryonic appendages, have attained their largest size; 

 they are capitate and will now decrease in size and disappear as the 

 anterior limbs develop and take upon themselves the function, 

 previously performed by the balancers. Plate 3, Figure 18. After 

 the branchiae have become as large as those represented in Figure 

 18, tiie development of the anterior limbs may be best studied by 

 cutting away the hinder pair of branchiae. The limb-processes 

 rapidly elongate, pointing backwards and a little downwards and 

 outwards; at first, they are simple rounded processes with an 

 unbroken outline until the length is two or three times the breadth. 

 When they have attained these dimensions a slight indentation is 



