274 G. E. COGHILL 



of the epithelium, which fits into the optic cup. Its margin con- 

 forms closely to that of the retina but there is a narrow space 

 between them. In the ventral portion of this space mesenchyme 

 cells occur. 



In the early flexure stage (fig. 74) the pigment layer has become 

 thinner, particularly at the margins, and its pigmentation has 

 increased considerably. The optic cup has greatly deepened. 

 The cell boundaries along the central border of the retina have 

 become less distinct and cells occur there which suggest the 

 nature of neuroblasts. Mitotic figures are still abundant in the 

 central portion of the ventricular margin of the retina. The 

 lens has acquired the form of a hemisphere, or shghtly in advance 

 of this, since the line of contact with the skin does not equal the 

 diameter of the lens. Mitosis is going on rapidly in the central 

 portion of the lens and there is a faint suggestion of a cavity in 

 it. The space between the lens and the retina has greatly in- 

 creased, owing, apparently, to the increased curvature of the 

 retina, and mesenchyme cells have pushed in almost to the center 

 of this space (fig. 74, Mes.). Mesenchyma is invading the space, 

 also, from dorsad. 



The retina of the coiled-reaction stage (fig. 76) has rounded 

 up into more nearly a spherical form. Cells which appear to 

 be neuroblasts are now conspicuous in the middle region of the 

 inner margin. The cells along the whole outer border (ventricu- 

 lar surface) are still proliferating rapidly by mitosis. While the. 

 lens is almost free from the skin, there is still a small area of 

 close contact with it. A distinct cavity has appeared in the lens, 

 on the outer side of which the epithelium is much the thicker. 

 The skin over the lens is still thick, and mitosis continues in the 

 lens. 



In embryos of the early swmming stage (fig. 78) the skin over 

 the lens has become reduced to a thin epitheUum, which, how- 

 ever, is still pigmented in a considerable degree. The lens, 

 though in contact with the skin, is perfectly detached from it. 

 The cavity in it has increased greatly in size, and the inner epi- 

 thelium is now much the thicker. The cells here are long and 

 columnar and their borders well defined. Mesenchyma extends 



