Oll tlie liirval flevelopment of Amia calva, 645 



the lenses are forraing; the two foremost gill slits are established, 

 and the tliird and Iburth already indicated. The pronephric duct, 

 p.n., passes tailward from its origin iramediately behind the bcginning 

 of the myelon ; unlike in Äcipenser, it is inconspicuous, taking its 

 Position elose to the trank, as in Teleosts. Notes on the early struc- 

 tural conditions of the pronephros, brain, hypophysis, chorda, digestive 

 tract and yolk are given below, pag. 654 — 668. 



Embryo surrounding 225 •'. 

 A late embryo of 142 hours is figured in PI. 9, Fig. 2. Its 

 trank is now a clearly defined ridge upon the egg's surface; it is no 

 longer demarked from the yolk region by the parietal zone as a marginal 

 trench. The pronephric ducts are most prominent at this stage, 

 separate from the trank region anteriorly, gradually converging tail- 

 ward and passing within the margin of the trank. The growth of 

 the tail now separates it from the egg. This condition may well be 

 Seen in PI. 9, Fig. 3, the ventral aspect of this stage. Here too may 

 be Seen the oatline of the sacking disc, s.d., its large size and 

 distinctly paired character^): its uplifted rim consists ou either side 

 of about 4 — 6 papilla-like sucking organs. The heart, too, is also 

 to be outlined in its thin walled and greatly dilated pericardial 

 Chamber ; it is straight and tabular, showing at either end its diverging 

 component elements. The mouth, m, is now a narrow, transversely 

 widened pit, partly concealed by the sucking disc. 



Embryo surroanding 240". 

 In this stage, 148 hours, PI. 9, Fig. 4, the trank has risen verti- 

 cally from the egg; the hinder trunk region, greatest in vertical 

 width, is now separate from the egg ; its terminal is blunt and rounded. 

 Four gill slits are now apparent outwardly; as yet, however, the 

 foremost two have alone broken through, and these but for a small 

 Proportion (V3) of their length' ventrally. The pericardial region is 

 now becoming reduced in size, losing its transparency. 



Embryo surrounding 290*^. 

 The region of the hinder trunk in this stage, 160 hours, is seen 



1) There can be no question that the figures of Allis, in: J. 

 Morph., '89, V. 2, p. 463 — 566, are inaccurate in this regard ; the paired 

 character of the sucking disc is shown in surface view and sections 

 from the time of its early appearance tili the time of its absorption 

 several weeks after hatching. 



