MORPHOLOGY OF TELEOSTEAN HEAD SKELEPON. 535 



by the presence of a well-developed pro-cartilagiuous palatine 

 process of the quadrate, exhibit the condition found in a 

 young salmon intermediate between Stohr's Stages III and 

 IV, of which the latter had been hatched some days. It 

 seems, therefore, that the simple quadrate without palatine 

 process, described by St5hr (83, p. 9) in salmon, and Pollard 

 (94, p. 69) in Gobius and Blennins, is either very transitory 

 or entirely omitted in the development of the stickleback. 

 These parts, which have been accelerated in development, 

 serve as a means of support to the mouth and operculum, or 

 as a means of attachment to the associated muscles. This 

 suggests that the acceleration may be due to these features 

 being of greater importance to this fish at an earlier stage 

 than to the salmon. In the stickleback there is practically 

 no alevin stage ; it hatches out on the ninth day, and by the 

 end of the eleventh its supply of yolk has been used up, and 

 it has begun to swim about fi-eely and to use its mouth 

 for foraging. It seems as though in this fish those elements 

 develop most rapidly during the embryonic period which 

 are most necessary to meet the immediate exigences of 

 larval life. 



Stage II. — The changes which have taken place in the 

 assumption of this stage are greater than any which take 

 place subsequently. All the elements in the adult pre- 

 formed in cartilage are already represented, and any further 

 development consists mainly in processes of remoulding 

 and ossifying. 



In the branchial apparatus all the arches are present, but 

 are still unsegmented (figs. 7 and 14). The first three {br. 1—3) 

 are equally developed, the fourth {br. 4) complete but reduced, 

 and the fifth incomplete and still further reduced (6r. 5). 

 Each complete arch has a flattened, expanded ventral end, 

 whilst its upper end is hook-shaped, and extends into the 

 tissue above the roof of the buccal cavity. Both these 

 features are lacking in the last arch. 



The copulare commune (fig. 14, br. b. 1—3), still unsegmented, 

 has extended forwards between the elements of the hyoid arch 



