loo 



AC1PENSKK. 



The epiblast at the sides of the brain constitutes a more or 

 less well-defined structure, which may be spoken of as a cephalic 

 plate (fig. 52 A, cp~). From this plate are formed the essential 

 parts of the organs of special sense. Anteriorly the olfactory pits 

 arise (fig. 52 B, Olp] as invaginations of both layers of the 



FIG. 52. EMBRYOS OF ACIPENSER BELONGING TO TWO STAGES VIEWED FROM THE 

 DORSAL SURFACE. (After Salensky.) 



Fb. fore-brain; Mb. mid-brain; Hb. hind-brain; cp. cephalic plate; Op. optic 

 vesicle; Auv. auditory vesicle; Olp. olfactory pit ; Ht. heart; Md. mandibular arch; 

 Ha. hyoid arch ; Br 1 . first branchial arch ; Sd. segmental duct. 



epiblast. The lens of the eye is formed as an ingrowth of the 

 nervous layer only, and opposite the hind-brain the auditory sack 

 (fig. 52 A and B, Auv} is similarly formed from the nervous 

 layer of the epiblast. At the sides of the cephalic plate the 

 visceral arches make their appearance; and in fig. 52 A and B 

 there are shewn the mandibular (Md}, hyoid (Ha) and first 

 branchial (Br'} arches, with the hyomandibular (spiracle) and 

 hyobranchial clefts between them. They constitute peculiar 

 concentric circles round the cephalic plate ; their shape being 

 due to the flattened form of the embryo, already alluded to. 



While the above structures are being formed in the head the 

 changes in the trunk have also been considerable. The meso- 

 blastic plates at the junction of the head and trunk become very 

 early segmented, the segments being formed from before back- 

 wards (fig. 50 B). With their formation the trunk rapidly 

 increases in 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 Elasmobranchs, as a solid outgrowth of the meso- 

 blast (fig. 5 1, Wg) ; but its anterior extremity becomes converted 

 into a pronephros (fig. 57, pr.n}. 



