THE ORGANS OF THE OUTER GERM-LAYER. 475 



process with a vascular loop, which makes its way into the primary 

 optic vesicle and its stalk (fig. 265). The vascular loop then begins 

 to send out new lateral branches ; likewise the connective-tissue 

 matrix, which is at first scanty, increases greatly and is characterised 

 by its extraordinarily slight consistency and its large proportion of 

 water (figs. 266, 267 g). There are also to be found in it here and 

 there isolated stellate connective-tissue cells ; but these disappear 

 later, and in their place occur migratory cells (leucocytes), which are 

 assumed to be immigrated white blood-corpuscles. 



There are two opposing views regai'ding the nature and develop- 

 ment of the vitreous body. According to KESSLER we have to do, 

 not with a genuine connective substance, but with a transudation, 

 a fluid, which has been secreted from the vascular loops ; the cells 

 are from the beginning simply immigrated white blood -corpuscles. 

 KOLLIKER, SCHWALBE, and other investigators, on the contrary, 

 regard the vitreous body as a genuine connective substance. Accord- 

 ing to SCHWALBE'S definition, to which I adhere, it consists of an 

 exceedingly watery connective tissue, whose fixed cells have early 

 disappeared, but whose interfibrillar substance infiltrated with water 

 is traversed by migratory cells. The vitreous body is afterwards 

 surrounded by a structureless membrane, the membrana hyaloidea, 

 which, according to some investigators, belongs to the retina, al- 

 though, according to the researches of SCHWALBE, this view is not 

 admissible. 



The vitreous body, which in the adult is quite destitute of blood- 

 vessels, is bountifully supplied with them in the embryo. They 

 come from the arteria centralis retince, the branch of the ophthalmic 

 aTtery that runs along the axis of the optic nerve. 



The arteria centralis retinae is prolonged from the papilla of the 

 optic nerve as a branch which is designated as the arteria hyaloidea. 

 This, resolved into several branches, runs forward through the 

 vitreous body to the posterior surface of the lens, where its numerous 

 terminal ramifications spread out in the tunica vasculosa, and at the 

 equator pass over on to the anterior face of the lens. During the 

 last months of embryonic life the vessels of the vitreous body, to- 

 gether with the nutritive membrane of the lens, undergo degenera- 

 tion ; they entirely disappear, with the exception of a rudiment of 

 the chief stem, which runs forward from the entrance of the optic 

 nerve to the anterior surface of the vitreous body, and during the 

 degeneration is converted into a canal filled with fluid, the canalis 

 hyaloideus. 



