FRONTAL SECTIONS OF HEAD, EMBRYO OF 20 MM. 329 



vascularized mesenchyma covered by ependyma. The choroid plexus protrudes 

 into the cavity of the brain in the same way in which the viscera- may be said 

 to protrude into the abdominal cavity. The cavity of the brain is bounded by 

 the brain-wall or ependyma, just as the abdominal cavity is bounded by the 

 peritoneum. The vascular tissue of the choroid plexus is outside of the cavity 

 of the brain, in the same way that the tissue of the kidney is outside the cavity 

 of the abdomen. Throughout life the choroid plexus springs, as it does from the 

 start, from the medial wall of the hemispheres, and it is only at that point that 

 it can receive its blood-supply. The lateral walls of the hemispheres, H, gradually 

 thicken as they continue ventralward, and on the ventral side of the brain form 

 in part the lateral boundary of the medial portion of the brain-cavity, as an especial 

 thickening of the brain-wall which projects far into the cavity. The thickening, 

 C.str, is the corpus striatum. Between the summit of the corpus striatum and 

 the choroid plexus is an open passage through which we may pass from the 

 median portion of the brain-cavity into the lateral ventricle, L.V. The passage 

 is the foramen of Munro, which we learn from this section is bounded above 

 by the choroid plexus, and below by the corpus striatum. On the dorsal and 

 middle sides of the hemispheres, the ectoglia, ec.gl, is already clearly differentiated. 

 There is, however, at this stage, no clear indication of the cortex cerebri, al- 

 though in the slightly older stages it will begin to develop by the accumulation of 

 neuroblasts immediately beneath the ectoglia. The notochord does not appear 

 between the brain and the pharynx, the section being too far forward. The noto- 

 chord stops near the hypophysis. The eyes are not cut quite symmetrically. They 

 show the lens, L, and retina, Ret, clearly and the left eye of the embryo shows 

 also the entrance of the optic nerve. On the right side of the embryo, near the 

 eye, are three areas which are somewhat more darkly stained than the surrounding 

 mesenchyma. These are the anlages of the muscles of the eye. They have not 

 yet been studied sufficiently to make their identification certain, but it seems prob- 

 able that the uppermost of these anlages, m.rec.sup, is the rectus superior, that the 

 middle one, m.retr.b, is the retractor bulbi, and that the lowest one, m.r.lat, is 

 the rectus lateralis. Until a reconstruction is . made these identifications can be 

 recorded as tentative only. The pharynx, Ph, appears as a yoke-shaped slit lined 

 throughout by entoderm. From its median ventral floor rises the great mass of 

 the tongue, Ton, over which the dorsal roof of the pharynx forms a closely fitting 

 arch. A portion of the epithelium of the tongue is loosened from the underlying 

 tissue, probably owing to defective preservation. Upon the lower side of the 

 tongue extend downward the anlages of the hyoglossal muscles, hy.gl, between 

 which are situated the lingual arteries, art. On either side, in the part of the 

 section corresponding to the mandible, appears Meckel's cartilage, Mk, a some- 

 what conspicuous and easily identified structure, owing to its dark staining. MeckeVs 

 cartilage is the primitive skeletal element of the mandibular arch, and is homol- 

 ogous with the cartilaginous jaw of the lower fishes. It is, for the greater part, 



