A HUMAN EMBKYO OF TWENTY-FOUR PAIRS OF SOMITES. LSI 



THE NERVOUS SYSTEM. 



The nervous system is represented by the brain, with its two optic vesicles 

 arid the beginnings of the trigeminal, aeustico-facial, glosso-pharyngeal, and vagus 

 nerves, and the medullary tube with its ganglionic crest. The dorsal wall of the 

 nervous system is placed just beneath the integument of the mid-dorsal line and 

 conforms to all its curvatures. The cavity of the tube is entirely closed off from 

 the outside, disregarding a longitudinal slit in the mid-dorsal line, which is clearly 

 artificial, and in this respect differs from that of Janosik's embryo, which showed 

 a small anterior neuropore. 



THE BRAIN. 



The three primary vesicles of the brain are easily recognizable in plate 2, 

 figure 1. The prosencephalon is large and bulbous, and is marked off from the 

 mesencephalon by a deep groove. Broad in front and in the region of the optic 

 vesicles, it gradually becomes narrower behind. Its cephalic end is rounded and 

 lies in close contact with the ectoderm; an actual fusion is at one place apparent, 

 marking probably the position of the closed anterior neuropore. There is no indi- 

 cation of a hemispheral division. The optic vesicles are attached to this portion 

 of the brain slightly ventral and anterior to its middle; they extend outward, 

 backward, and slightly dorsal ward. There is as yet no definite indication of a 

 division of the prosencephalon into diencephalon and telencephalon. A slight 

 rounded protuberance of the ventral wall behind the points of attachment of the 

 optic vesicles probably marks the beginning of the infundibulum. 



The mesencephalon is a small wedge-shaped portion of the brain-tube. Of 

 the grooves which mark it off from the prosencephalon in front and the rhomb- 

 encephalon behind, the anterior is deeper; the posterior groove is faint dorsally, 

 but ventrally it ends in a deep notch. The mesencephalon is much narrower from 

 side to side than the prosencephalon. Its antero-posterior dimension is only a 

 trifle greater than those of the neuromeres of the rhombencephalon immediately 

 behind it. In marked contrast to this is the much longer and larger mesencephalon 

 of the Robert Meyer embryo, as modeled by Thompson 45 . Somewhat similar to 

 it, however, is that of Ingalls's embryo. 



The rhombencephalon is elongated and flattened laterally. As seen from the 

 side it is slightly curving, its dorsal wall being convex. 



MEDULLARY TUBE. 



The spinal part of the medullary tube extends from the rhombencephalon 

 to the tip of the tail, gradually tapering from above downward. It is ovoid in 

 section, the lateral walls being thicker than the dorsal and ventral walls. The 

 dorsal wall is thinnest and lies almost in contact with the covering ectoderm. 



NEUROMERES. 



Both the rhombencephalon and spinal portion of the medullary tube are 

 marked off by transverse grooves into a series of segments, the so-called "neuro- 

 meres." These begin at the cephalic end of the rhombencephalon and continue 

 downward through the medullary tube. The first six neuromeres are narrow. 



