Developmental history of primary segments of the vertebrate head. 441 



Expliuiatioi) of Plates. 



Plate 28—30. 



List of Reference Mai'ks. 



7, 2, 5, 4 etc. metameric segments 



a, &, c, d etc. external grooves that form dividing lines 

 between segments 



a' , h\ c', d' etc. internal dividing lines between seg- 

 ments 



III, IV, V etc. cranial nerves 



opt.n optic nerves 



olf.pt olfactory pit 



au.vs auditory vesicle 



s' , s" etc. somites 



inf infundibulum 



no. CO notochord 



gas.ga G-asserian ganglion 



cil.ga ciliary ganglion 



po. com posterior commissure 



r dorsal groove that divides the primary fore-brain 

 into prosencephalon and thalamencephalon 



S dorsal groove that divides the thalamencephalon into 

 an anterior and a posterior division 



e right epiphysial vesicle 



e' left epiphysial vesicle. 



Plate 28. 



All figures are of Salmo purpuratus (Pallas) except Fig. 9, which 

 represents Salmo fario (Mitch.). All are X ^0 diameters. 



Fig 1. Encephalon of a dissected embryo with 29 somites, 19 days 

 old. Right surface view. Segments 1, 2 and 3 represent the fore-brain, 

 and 4 and 5 the mid-brain. The Anlage of the cerebellum is repre- 

 sented by segment 6. 



Fig. 2. Dorsal view of the same encephalon that is represented 

 in Fig. 1. The five segments of the fore- and mid-brain show in this 

 view. 



29* 



