CYCLOPIA IX THE HUMAN EMBRYO. 



25 



Sup. Ob. 



appears to be normal, but it is markedly dissociated. Below the upper lumbar 

 region the spinal cord is wholly wanting and the spinal canal is filled with meso- 

 dermal tissue which is rich in blood-vessels. Where the cord is missing most 

 of the spinal nerves still remain, and many dorsal ganglia can be made out, show- 

 ing that the changes in the central nervous system in this region took place after 

 the spinal nerves had been developed from it. 



Most of the epidermis is intact, but it is broken through at the back of the 

 head, where there is found an extensive ulcer or necrotic mass, which is very rich 

 in blood-vessels and involves the walls of the brain but does not reach into its 

 ventricle. At the highest point of the head the epidermis has developed into a 

 papilliform body, and below this there is a large necrotic area in which there is a 

 great quantity of yellow pigment granules. 



The mouth is closed completely, although 

 the alimentary canal, from the mouth to the 

 stomach, is open and appears normal. The 

 intestine is matted together; the cloaca and 

 anus are obliterated. The epithelium of the 

 upper portion of the intestine gives rise to 

 marked growths into the matted mass. 



The thoracic region, liver, and vascular 

 system have undergone practically no change. 

 The extensive dissociation of the tissues 

 throughout the embryo has caused an exten- 

 sive destruction and arrest in the develop- 

 ment of the muscular system. This is marked 

 by all kinds of secondary changes in the con- 

 nective tissue, especially in that of the skin, 



Where the Change is pronounced, aS may be FIG. 3. Diagrammatic reconstruction of the eye and 



QPPTI in nlafp 9 fimirp 4 Tn thi! rPO'inn tllP the surrounding structures of embryo No. 201. X25. 

 86611 in piatC Z, nglire imS leglO The cranial nerves are marked with Roman numerals. 



change is so great that it obliterates entirely ]? ^ st branch . ! fi f th n r , ve; s - rudimentary snout; 



E. M., eye-muscle; Sup. Ob., superior oblique muscle. 



the external auditory canal. 



A reconstruction of the brain and upper part of the spinal canal enabled me 

 to determine with greater precision the parts of the brain which are within the 

 head of this embryo. Subsequently it was possible to find the remnants of the 

 ganglion of the eighth nerve; then that of the fifth nerve was determined. Their 

 position is shown in the profile diagram of the outlines of the brain and these 

 nerves (fig. 2). It at once becomes apparent that the portion of the brain extend- 

 ing up in the necrotic cap on the top of the head consists of the midbrain and 

 hindbrain. With this idea in mind it was possible to answer the question in the 

 serial sections, as the walls of the midbrain are thick and the ventricle relatively 

 small, and the walls of the hindbrain are thin and its ventricle relative large. 

 The pointed process which reaches towards the ganglion of the fifth therefore 

 represents the pontine flexure of the medulla. It is now easy to interpret the 



