88 



ACIPENSER. 



length. At their outer border the segmental duct (fig. 50 B, and 

 fig. 52 A, Sd) is very early established. It is formed, as in Elasmo- 

 branchs, as a solid outgrowth of the mesoblast (fig. 51, Wg) ; but its 

 anterior extremity becomes converted into a pronephros (fig. hl,pr.n). 



Before hatching, the embryo has to a small extent become folded 

 otf from the yolk both anteriorly and posteriorly; and has also 

 become to some extent vertically compressed. As a result of these 

 changes, the general form of its body becomes much more like that 

 of an ordinary Teleostean embryo. 



The general features of the larva after hatching are illustrated 

 by figs. 53, 54 and 55. Fig. 53 represents a larva of about 7 mm. 

 and fig. 54 a lateral and fig. 55 a ventral view of the head of a larva 

 of about 11 mm. 



There are only a few points which call for special attention in the 

 general form of the body. In the youngest larva figured the ventral 

 part of the hyomandibular cleft is already closed : the dorsal part 

 of the cleft is destined to form the spiracle (sp). The arch behind 

 is the hyoid : on its posterior border is a membranous outgrowth, 



Fig. 53. Larva of Acipensek of 7 mm., shortly after hatching. 

 ol. olfactory pit ; op. optic vesicle ; sp. spiracle ; hr.c. branchial clefts ; an. anus, 



which will develop into the operculum. In older larvse, a very 

 rudimentary gill appears to be developed on the front walls of the 

 spiracular cleft (Parker), but I have not succeeded in satisfying myself 

 about its presence ; and rows of gill papillse appear on the hyoid 

 and the true branchial arches (figs. 54 and 55, g). The biserially- 

 arranged gill papillse of the true branchial ai-ches are of considerable 

 length, and are not at first covered by the operculum ; but they do 

 not form elongated thread-like external gills similar to those of the 



' Elasmobranchii. 



The oral cavity is placed on the ventral side of the head, it has 

 at first a more or less rhomboidal form. It soon however (fig. 55) 

 becomes narrowed to a slit with projecting lips, and eventually 



[becomes converted into the suctorial mouth of the adult. The most 

 remarkable feature connected with the mouth is the development of 

 provisional teeth (fig. 55) on both jaws. 



These teeth were first discovered by Knock (No. 88). They do not appear 

 to be calcified, and might be supposed to be of the same nature as the horny 

 teeth of the Lamprey. They are however developed like true teeth, as a 



