Ou the larval developinent of Amin calva. 647 



the left side of tlie trunk; the vitelline vessels become a ])rominent 

 feature of the dorsal aspect of the yolk sac; the sucking diso is in 

 this stage relatively at its Kargest size. 



Larva of the secoud day (PL 9, Fig. 8). 

 The straightening of the trunk takes place during the second day, 

 a change accompanied with a great development of the continuous 

 unpaired fin; tliis now surrounds the entire hinder trunk, and in the 

 caudal region develops a noticeable heterocercy. At the same time 

 the pectoral fin becomes conspicuous, typically fin-fold-like in character, 

 resembling the corresponding stage in the fin growth of the older 

 Ganoids; it coutains about seven (metanieral) mesoblast buds. An 

 elongatiou of the yolk sac accorapanies these changes ; and the entire 

 head region has become distiuct from the sac and greatly enlarged. 

 At the side of the head the dermal operculum is enclosing the gill 

 slits; the auditory vesicle is clearly defined; and the formation of 

 the lens has taken place. There is a general loss of the transparency 

 of the head and anterior trunk region; pigraent patches become 

 evident; the brain vesicles can uo longer be distinguished through the 

 head wall. The blood has now acquired a reddish colour and the 

 pulsation of the heart can be followed; the long caudal vessel 

 forms a broad band at the ventral margin of the somites of the trunk. 

 Mouth and anus are established; the pronephric ducts are to be 

 traced above the line of the bind gut, and may be seen to unite just 

 above the anus; their common opening to the exterior appears to be 

 on the point of formation. The head surface shows a faint out- 

 mapping of the sensory canals; the nervus lateralis vagi is seen to 

 have pushed its way hindward along the side of the trunk to near 

 the pectoral region (cf. Allis, op. cit.). 



Larva of the third d ay (PI. 9, Fig. 9). 

 The advances in this stage iuclude: pigmentation, increased size 

 and the distinctness from the yolk sac of head and trunk; enlarge- 

 ment of the pectoral fins and of the dermal operculum, which has 

 now entirely overlapped the region of the gills; the U-shaped con- 

 dition of the heart ; the Separation of the chin region from the yolk ; 

 the distinct appearance of the thickened tracts of sensory epiblast in 

 the head surface; the progress of the nervus lateralis vagi to that 

 portion of the mid-lateral line of the trunk immediately above the 

 bind margin of the yolk sac. 



