TELEOSTET. 



77 



the embryo continues to grow, remaining however closely 

 applied to the yolk-sack, round which it curls itself to an extent 

 varying with the species (vide fig. 34). 



The general growth of the embryo during the later stages 

 presents a few special features of interest. The head is remark- 

 able for the small apparent amount of the cranial flexure. This 

 is probably due to the late deve- 

 lopment of the cerebral hemi- 

 spheres. The flexure of the floor 

 of the brain is however quite as 

 considerable in the Teleostei as in 

 other types. The gill clefts deve- 

 lop from before backwards. The 

 first cleft is the hyomandibular, 

 and behind this there are the 

 hyobranchial and four branchial 

 clefts. Simultaneously with the 

 clefts there are developed the 

 branchial arches. The postoral 

 arches formed are the mandibular, 

 hyoid and five branchial arches. In the case of the Salmon all 

 of these appear before hatching. 



The first cleft closes up very early (about the time of 

 hatching in the Salmon) ; and about the same time there springs 

 a membranous fold from the hyoid arch, which gradually grows 

 backwards over the arches following, and gives rise to the 

 operculum. There appear in the Salmon shortly before hatching 

 double rows of papillae on the four anterior arches behind the 

 hyoid. They are the rudiments of the branchiae. They reach 

 a considerable length before they are covered in by the opercu- 

 lar membrane. In Cobitis (Gb'tte, No. 64) they appear in young 

 larvae as filiform processes equivalent to the external gills of 

 Elasmobranchs. The extremities of these processes atrophy; 

 while the basal portions become the permanent gill lamellae. 

 The general relation of the clefts, after the closure of the 

 hyomandibular, is shewn in fig. 35. 



The air-bladder is formed as a dorsal outgrowth of the alimentary tract 

 very slightly in front of the liver. It grows in between the two limbs of the 

 mesentery, in which it extends itself backwards. It appears in the Salmon, 



FlG. 34. VIEW OF AN ADVANCED 

 EMBRYO OF A HERRING IN THE 

 EGG. (After Kupffer.) 



oc. eye ; ht. heart ; hyv. post-anal 

 vesicle ; ch. notochord. 



