GANOID EL 



87 



is something like that of an ordinary embryo slit open along the 

 ventral side and then flattened out. Organs which properly belong 

 to the ventral side appear on the lateral parts of the dorsal surface. 



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

 DORSAL SURFACE. (After Salensky. ) 



Fb. fore-brain ; Ml>. mid-brain; lib. hind-brain; cp. cephalic plate; Op. 

 vesicle; Auv. auditory vesicle; Olp. olfactory pit; Ht. heart; Md. niandilml;n 

 arch; Ha. hyoid arch; />'/'. first branchial arch ; Sd. segraeutal duct. 



Owing to the great forward extension of the yolk the heart (fig. 52 B) 

 appears to be placed directly in front of the head. 



Even before the formation of the medullary canal the cephalic 

 portion of the nervous system becomes marked out. This part, after 

 the closure of the medullary groove, becomes divided into two (fig. 

 50 B), and then three lobes the fore-, the mid-, and the hind-brain 

 (fig. 52, A and B). From the lateral parts of the at first undivided 

 fore-brain the optic vesicles (fig. 52 B, Op) soon sprout out ; and in 

 the hind-brain a dilatation to form the fourth ventricle appears in 

 the usual fashion. 



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 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 hy o mandibular 

 (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 backwards 

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



