EMBRYO OF 24 MM. 263 



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 to- 

 gether. 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, Yit, 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 proto- 

 plasmic threads of the mesenchyma. 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 hya- 

 loid membrane are a number of small blood-vessels, more numerous than those 

 elsewhere in the vitreous humor, and forming a fairly distinct vascular mem- 

 brane 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 recon- 

 struction. 



Median Sagittal Section (Fig. 151) . — 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 almost as 

 great as that occupied by the rest of the body. The great size of the head at this 

 stage is characteristic. Attention is especially directed to the sharp angle which 

 the medulla oblongata, Md. oh, makes with the spinal cord, Sp. c, and to the very 

 great bend formed by the floor of the mid-brain, Ar. hah, in consequence of which 

 the floor of the hind-brain above the medulla oblongata and the floor of the fore- 

 brain are brought quite close together and run in almost parallel directions. The 

 cavity of the brain is very large. Its walls in the median plane are, for the most 

 part, thin. From the roof of the diencephalon, Dien, there runs off a small eva- 

 gination, Ephys, a shallow pocket or diverticulum of the medullary wall. . It is 

 the anlage of the epiphysis or pineal organ of the adult. It is an important land- 

 mark in the topography of the brain, for its position is always at the extreme 

 limit of the fore-brain. In the wall of the mid-brain, behind the epiphysis, for 



