STUDY OF SECTIONS OF EMBRYO OF 24 MM. 333 



layer corresponding to the ectoglia, and within a broad, nucleated zone. The 

 mitotic figures are found only next to the surface of the retina nearest the pig- 

 ment layer. Since the space between the pigment layer" and the retina corresponds 

 to the cavity of the brain, it is evident that the position of the mitotic figures 

 is homologous with their position in the medullary wall elsewhere. The section 

 of the lens clearly reveals its vesicular structure. The external wall of the lens 

 vesicle, L', is a comparatively thin epithelial layer which stains quite readily and 

 therefore stands out clearly in the section. Toward the edges of the lens the outer 

 layer slightly thickens and then- passes over quite abruptly into the inner layer of 

 the vesicle, L" ', which is very thick and constitutes by far the greater part of the 

 bulk of the organ and gives to the lens its characteristic shape. The outer and 

 inner walls of the lens are in close contact so that there is no actual cavity present. 

 The epithelial cells of the inner wall have elongated enormously, so much that 

 they might perhaps already be. termed "fibers." Each cell is supposed to extend 

 through the entire thickness of the inner wall. The nuclei are placed somewhat 

 irregularly in the middle portion of the long -cells so that they constitute a more 

 or less distinct band in the section. Toward the edge of the lens the nuclear band 

 becomes more distinct, and where the inner wall merges into the outer, the band 

 becomes narrow and the nuclei are much crowded together. The nuclei of the lens 

 fibers are oval, being slightly elongated in the same direction as the fibers, and 

 each nucleus contains usually a distinct nucleolus. Between the lens and the retina 

 is the vitreous humor, Vit, which has become quite voluminous. It contains a few 

 mesenchymal cells and a few small blood-vessels, and when examined with a high 

 power it is, seen to be permeated by a fine network which is probably to be 

 interpreted as a modification of the protoplasmic threads of the mesenc.hyma. There 

 are also a very few cells of rounded form and distinct outline, with a single small 

 granular nucleus, which are probably leucocytes. Against the surface of the lens 

 there is a delicate homogeneous hyaloid membrane, which can usually be better 

 seen where by shrinkage it has been loosened from the surface of the lens, as 

 is apt to occur. Against the hyaloid membrane are a number of small blood-ves- 

 sels, more numerous than those elsewhere in the vitreous humor, and forming a 

 fairly distinct vascular membrane around the lens. The membrane, tu.v, is called 

 the tunica- vasculosa lentis. The blood-vessels of the vitreous humor are chiefly, 

 possibly at this stage exclusively, branches of the central artery of the retina. The 

 artery enters the eye through the optic nerve, and sends branches throughout the 

 vitreous humor. The space originally occupied in the humor by the stem of the 

 central artery persists, and is called the hyaloid canal. The muscles of the eye 

 are already differentiated, but their relations cannot be properly understood without 

 a reconstruction. 



Median Sagittal Section (Fig. 224). The section figured is very nearly median 

 for the region of the head, but in the body it passes to the left of the median 

 plane. The area occupied in the section by the neck and head of the embryo is 



