DEVELOPMENT OP PETROMYZON PLUVFATILTS. 201 



suggests, connected with the openings of the ducts of the pro- 

 nephros, for this gland is completed and seems capable of 

 functioning long before any food could find its way through 

 the mid-gut, or indeed before the stomodseum has opened. 



The stomodseum has a very early origin ; it commences on 

 the fifteenth day as an invagination of ectoderm against the 

 blind anterior end of the fore-gut. This gradually deepens 

 and attains a very large size, partly due to great development 

 of the upper lip, which grows forward and downward to con- 

 stitute the large hooded structure which is so characteristic of 

 the Ammocoete. The greater part of this hood consists of 

 simple muscle-fibres which interlace and cross one another in 

 a diagonal direction. The lower lip does not reach so far for- 

 ward as the upper (figs. 34 and 35). About the twentieth day 

 the velum begins to appear in the posterior angle of the sto- 

 modseum. This structure is formed by two grooves which 

 gradually deepen and cut oflp a flap of tissue on each side of the 

 middle line. These two grooves, shown in fig. 27, are not very 

 deep. The tissue between them is broken through the next 

 day so that the two lateral folds that remain are covered on 

 their anterior face by epiblast, and on the greater part of their 

 posterior face by hypoblast (fig. 28). Subsequently the meso- 

 blast in these two flaps develope into muscle-fibres, and in the 

 young larva a constant current is kept up by them, passing in 

 at the mouth and out at the gill-clefts. This current is easily 

 demonstrated by the aid of a little Indian ink suspended in 

 the water. 



On the twenty-third day two tentacles begin to grow out 

 from the under surface of the upper lip, one each side of the 

 middle line ; a little later two more appear on the sides, but 

 placed more posteriorly ; later still two more appear behind the 

 level of the last ; these are situated at the junction of the lower 

 lip with the upper. Finally, a median tentacle appears in the 

 ventral middle line. This last is far longer than the others 

 and from its base a ridge, which is at first low, but increases in 

 height posteriorly, extends back between the ventral portion of 

 the ciliated ring (figs. 40 and 41). The number of tentacles 



15 



