468 EMBRYOLOGY. 



by means of a slender stalk only (fig. 264 A si). They posses 

 spacious cavities within, which are connected with the system of 

 brain-ventricles through the narrow canal of the stalk of the optic 

 vesicle. In many Vertebrates, in which the central nervous system 

 is formed as a solid structure, as in the Cyclostomes and Teleosts, 

 the optic vesicles are also without cavities; these do not make 

 their appearance until the central nervous system becomes a 

 tube. 



Since the brain is for a long time separated from the primitive 

 epidermis by only an exceedingly thin sheet of connective tissue, 

 the primary optic vesicles at the time of their evagination either 

 apply themselves directly to the epidermis, as in the case of the 

 Chick, or are separated from it by only a very thin intervening 

 A ln.yer, as in Mammals. 



fc Upon each optic vesicle- 



/s can be distinguished a 



^"\^"^ lateral, a median, an upper 



and a lower wall. I 

 designate as lateral that 

 surface which reaches the- 



Fig. 264,-Two diagrams illustrating the development epidermis at the Surface 



ofthee y e - of the body, as median 



A, The primary optic vesicle (an), joined by a hollow . . 



stalk (Bt) to the between-brain (zh), is invaginated the Opposite Wall joined 

 as a result of the development of the lens-pit (Ig). with thp <tfallr of thft rmtip 



B, The lens-pit has become abstracted to form a lens- 6 PtlC 



vesicle (Is). From the optic vesicle has arisen the VCSlcle, and finally as lower 

 optic cup with double walls, an inner (i&) and an ^^ nTlp whifb lip<5 nn 

 outer (06) ; 1st, stalk of the lens ; gl, vitreous body. 



a level with the floor of 



the between-brain. These designations will be useful in acquainting 

 ourselves with the changes which the form of the optic vesicle 

 undergoes during its imagination, which occurs at two places, namely, 

 at its lateral and lower surfaces. Om of the imaginations is connected 

 with the development of the kns, the other with the formation of the 

 vitreous bcify. 



The frsl fundament of the lens appears in the Chick as early as 

 the second day of incubation, in the Rabbit about ten days after 

 the fertilisation of the egg. At the place where the epidermis 

 passes over the surface of the primary optic vesicle (fig. 264 A Ig). 

 it becomes slightly thickened and invaginated into a small pit (lens- 

 pit). The pit, by its deepening and by the approximation of its- 

 edges until they meet, is converted into a lens-vesicle (fig. 264 E Is),. 

 which for a time preserves its connection with its parental substra.- 



