TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 187 



central cavity of the brain. The inner of the two layers forms the retina proper 

 and is considerably thickened. The outer layer is quite thin and is already 

 quite abundantly laden with pigment granules. At the edge of the cup the pig- 

 ment layer passes over uninterruptedly into the thick retina layer. In the cav- 

 ity of the optic cup lies the vesicular lens, L, which arose from an evagination of 

 the overlying ectoderm. The vesicle is, however, now completely separated 

 from the layer which produces it. It has at this stage a very large cavity, and in 

 adjacent sections it can be readily seen that the inner side or that toward the 

 brain is already thickening and changing its character so as to form the main part 

 of the adult lens. The thickening depends chiefly upon the rapid and enor- 

 mous elongation of the epithelial cells of this part of the vesicle, so that they are 

 transformed into the so-called fibers of the adult lens, each adult fiber being a 

 single epithelial cell. 



Section through the Second Gill Cleft and Oral Fissure. — The level of this sec- 

 tion is such that the head is cut separately and appears in section without connec- 

 tion with the body of the embryo. The space between the head piece and body 

 piece, O. F, may be designated as the oral fissure, since it is into this space that 

 the mouth opens. In general there is considerable resemblance between this and 

 the section last described, but in the present section the eyes have disappeared 

 and we get the first indications of the nasal pits, Olf. That on the left side of the 

 body shows a trace of the cavity of the pit. The posterior part of the pharynx, 

 Ph, is cut in the section instead of the anterior part, as in figure 116. The first 

 gill cleft does not show, but the second cleft, cl. II, does. It lies posterior to the 

 first cleft and, therefore, appears higher up in the figure. The spinal cord, Sp. c, 

 shows the same general structure as in the previous section. On either side of it 

 may be seen the small and inconspicuous root of the eleventh or accessory nerve. 

 It could not be properly represented in the figure. Some distance below the 

 cord lies the small circular section of the notochord, which differs so slightly in 

 staining from the surrounding mesenchyma that it cannot be well made out 

 without the use of a higher magnifying power. It is enclosed by a distinct mem- 

 brane wliich is thick enough to produce a double outline, and contains a consid- 

 erable number of scattered nuclei, which are, however, nowhere much crowded. 

 The nuclei are round in form, decidedly larger than those of the surrounding 

 mesenchyma, granular, and containing each several more conspicuous, darkly 

 staining granules. There is a very slight gathering of mesenchymal cells about 

 the notochord, as if to form the anlage of a sheath. Just below the notochord 

 there is a broad band of somewhat darker staining, due to a greater condensation 

 of the mesenchyma in that region, and this condensation represents the beginning 

 of the formation of the vertebral structures. On either side we find the trans- 



