262 



ANATOMY OF VERTEBRATES. 



136 



the back part of the circuhir sac, and, in the course of its muta- 

 tion into eyeball, lines its posterior part with the layer called 

 retina, interrupted only by the cicatrix of the inferior and rapidly 

 blended ends of the primitive eye-sac. The transparent layer 

 covering the fore-part of that sac and the inclosed lens is meta- 

 morphosed into cornea. Other layers of the sac are differentiated 

 into choroid, ciliary processes, iris ; and a fold of the vascular 

 layer protrudes through the choroid fissure as a persistent struc- 

 ture in birds, in which the ^ pecten ' significantly marks a curious 

 step in the developement of the eye in all Vertebrates. Of the 

 appendages of the eye the membrana nictitans, fig. 137, «, is 

 the first to appear, the lower lid and then the upper lid follow. 

 It is a mistake to speak of the labyrinth or eyeball as being- 

 formed by the integument, or beginning as ^ cutaneous follicles,' 



for the structures of the skin are not 

 differentiated when they first appear ; 

 a layer of cellular or primitive blastemal 

 tissue represents the integument, and a 

 greater number of cells is aggregated at 

 the points which tend inward to meet 

 the productions from the nervous centres. 

 After the essential oro-ans of sense are 

 established, then is the skin developed 

 and modified more or less for their pro- 

 tection, forming the outer ear and the 

 eyelids : but both passages are closed by 

 transparent membranes, as ' ear-drum ' 

 and ^ cornea.' Only in the case of the 

 olfactory organ does the primitive de- 

 pression, fig. 135, r, retain its outlet, 

 and in the bird and other air-breathers, 

 it also communicates with the air-pas- 

 sage : ha\dng the tegument superadded 

 and modified, in most, as external nostril 

 and nose. 



As in the Lizard and Snake (Vol. I. 

 fig. 444), so in the Bird, the four verte- 

 bral segments constituting the head arc 

 shown by the embryological characters 

 and course of formation of the ^ maxil- 

 lary ' arch, figs. 135, 136, a, the '^man- 

 dibular arch,' ib. Z», the ' hyoidean arch,' 

 ib. c, and the scapular arch, ib. d. The transitory branchial arte- 



Primitive blood-vesrels of embryo 

 Bird second day. lv. 



