258 STUDY OF PIG EMBRYOS. 



the falx, and it is covered by a continuation of the wall of the hemispheres. The 

 covering layer of the fold is much thinner than any other portion of the brain- 

 wall shown in the section, and shows no differentiation into layers. It retains 

 throughout life an epithelial character and is already to be termed ependyma. 

 The ependyma of the two folds is connected across the median line, and it forms 

 the median dorsal boundary of the cavity of the fore-brain. The two folds are the 

 anlages of the lateral choroid plexus. They are destined to grow much in size 

 and in complexity of form, but they always remain morphologically what they 

 now are, 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 ventralwards, 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. sir, 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 ventri- 

 cle, 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, although in the slightly older stages it will begin to develop by 

 the accumulation of neuroblasts immediately beneath the ectoglia. The noto- 

 chord does not appear between the brain and the pharynx, the section being too 

 far forward. The notochord 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 probable that the uppermost of these anlages, 

 m. rec. sup, is the rectus superior, that the middle one, m. rctr. b, is the retractor 

 bulbi, and that the lowest one, m. r. lot, is the rectus lateralis. Until a recon- 



