On the larval develoimicut of Ainia calva. 



657 



curs, as a nick in the margin of the ventral unpaired fin, PI. 9, Fig. G p. 

 A siniilar condition occiirs in Acipenser and Lepidosteus. 



A post-anal gut is suggested in the sagittal section of the tail of 

 a late embryo (slightly earlier than that figured in the present paper in 

 PI. y, Fig. 1) shovvn in Fig. G PAG. But throughout the early stages 

 in the growth of the tail region there is no trace of a neurenteric 

 canal. It will be remembered that in Amia the neural axis, as in 

 Teleosts, is formed as a solid keel-like insinking of the ectoderm; 

 and that its lumen is acquired at a later period — later than the 

 blastopore's closure. A study of serial sections demonstrates that this 

 lumen extends hindward into the outgrowing tail bud, as in Fig. G sepa- 



/VT 



Ch 



(^ AG l G 



Fig. G. Sagittal section of the tail region of embryo snrrounding about 195 " of 

 egg's circumference. BO body cavity, CR notochord, E, E inner and outer layer of 

 epidermis, / intestine, L lumen of neural tube, NT neural tube, PAG region of post- 

 anal gut, U undiflferentiated tissue of tail. ^ 



rate I and terminates blindly in the undiflferentiated tissue of the caudal 

 mass, TJ. No traces of a connection have been found between the 

 cavity of the medullary tube and the Virtual lumen of the post-anal 

 gut, FAQ. A similar condition has been described in Lepidosteus^ 

 although there can be no doubt that a truly shark-like neurenteric 

 canal occurs in Acipenser. 



The relation of the pronephric ducts to the hind gut and the 

 Position of their common opening in the region immediately behind 

 the anus have already been noted. In their tailward growth the 

 ducts, keeping close to the intestine, have been found to remain 

 separate tili they have attained a position slightly above and in front 

 of the anal region (PI. 9, Fig. 6); in this position they fuse, and, 



ZooL Jahrb. IX. Abth. f. ?yst. 43 



