9 2 



GENERAL DEVELOPMENT. 



line of insertion of the velum appears to me to represent the mandibular 

 arch. The grounds for this view are the following : 



(1) The structure in question has exactly the position usually occupied 

 by the mandibular arch. 



(2) There is present in late larvae (about 20 days after hatching) an 

 arterial vessel, continued from the ventral prolongation of the bulbus 

 arteriosus along the insertion of the velum towards the dorsal aorta, which 

 has the relations of a true branchial artery. 



On the ventral aspect of the branchial region is placed a sack 

 (figs. 42, h, and 43, tJi), which extends from the front end of the 

 branchial region to the fourth cleft. At first it constitutes a 

 groove opening into the throat above (fig. 44), but soon the 

 opening becomes narrowed to a pore placed between the second 

 and third of the permanent branchial pouches (fig. 43, t/i). In 

 Ammoccetes 1 the simple tube becomes divided, and assumes a 

 very complicated form, though still retaining its opening into 

 the branchial region of the throat. In the adult it forms a 

 glandular mass underneath the branchial region of the throat 

 equivalent to the thyroid gland of higher Vertebrates. 



On the ventral aspect of the head, and immediately in front 

 of the mouth, is placed the olfactory pit (fig. 43, <?/). It is from 

 the first unpaired, and in just-hatched larvae simply forms a 

 shallow groove of thickened epiblast at the base of the front of 

 the brain. By the stage represented in fig. 43 the ventral part 

 of the original groove is prolonged into a pit, extending back- 

 wards beneath the brain nearly up to the infundibulum. 



On the side of the head, nearly on a level with the front end 

 of the notochord, is placed 

 the eye (fig. 43, op}. It is 

 constituted (figs. 45 and 46) 

 of a very shallow optic cup 

 with a thick outer (retinal) 

 layer, and a thin inner cho- 

 roid layer. In contact with 

 the retinal layer is placed 

 the lens. The latter is form- 

 ed as an invagination of the 



FIG. 44. DIAGRAMMATIC TRANSVERSE 

 SECTIONS THROUGH THE BRANCHIAL REGION 



OF A YOUNG LARVA OF PETKOMYZON. (From 

 Gegenbaur ; after Calberla.) 



d. branchial region of throat. 



1 Schneider (No. 85) states that in the full-grown Ammocretes the opening is situ- 

 ated between the third and fourth pouches. This is certainly not true for the young 

 larva. 



