7 

 In embryos of 4, 5, and 6 mm, we witness the origin of the eye. 

 The primary optic vesicle gradually invaginates to form the optic 

 cup, the overlying body ectoderm thickens and bends inward to give 

 rise to the lens vesicle. Cup and lens are in close contact through- 

 out; no mesoderm is found between them (Figure 6). The cavity 

 of the vitreous body, the posterior chamber of the eye, is absent in 

 the younger specimens, but appears as a narrow slit between future 

 retina and lens in the 6 mm embryo. The development thenceforth 

 is very rapid, and in the 11 mm embryo, the structure of the eye 

 may be described thus : the optic cup is complete, consisting of an 

 inner layer, the retina, and an outer, the pigment layer. The former 

 shows two distinct regions, a cellular, and the so-called mantle layer. 

 Nerve fibers and pigment are still absent. The lens vesicle has sepa- 

 rated from the body ectoderm. The choroid fissure is wide open, 

 and the surrounding mesoderm is pressing up into the fissure (Fig- 

 ure 3), but has not yet penetrated into the optic cup. The optic 

 stalk, an open tube with wide lumen, can easily be traced to the 

 diencephalon. 



The cavity of the vitreous body is still narrow. It contains no 

 blood vessels and only a few cells scattered here and there. The 

 origin and the nature of these cells have not been satisfactorily ex- 

 plained. Some authors, as v. Lenhossek, consider them the rem- 

 nants of the thin layer of mesoderm originally found between the 

 optic vesicle and the body ectoderm, and carried into the optic cup 

 by the invaginating lens placode. Others, among whom are See- 

 felder, Mavas, and Magitot, maintain that these cells have migrated 

 from the retinal mantle layer into the vitreous body. Seefelder 

 proposes to call them ectodermal vitreous body cells (ektodermale 

 Glasskorperzellen) , and believes them to be identical with similar 

 cells found later in large numbers in the mantle layer of the retina. 

 Mavas and Magitot, moreover, are of opinion that, passing through 

 a process of degeneration and disintegration, these cells contribute 

 to the production of the liquid parts of the vitreous body. Wolfrum 

 believes some to be real mesoderm cells with vaso-f ormative function, 

 others he prefers to call ectodermal cells. The difference of opinion 

 may, to some extent, be owing to the difference in the material used. 

 In the material which formed the subject of bur investigation, the 

 cellular elements are exceedingly rare, scarcely half a dozen being- 

 counted in as many complete series of sections. In the' eye. Of the 

 pig, in which no mesoderm is carried into the optic cup by the lens 



