TERATOGENESIS. 619 



ORIGIN or MALFORMATIONS IN THE REGION OF THE NEURAL TUBE. 



Normally the neural tube is formed from a band of ectoderm extending 

 along the dorsum of the embryonic disk. The ectodermal band becomes 

 thickened, a groove appears along the middle line and the margins are raised 

 above the surface of the embryo, forming the neural groove. The margins of 

 the band continue to push upward and finally meet and fuse with each other 

 throughout their entire length in the middorsal line. The surface ectoderm 

 then breaks away from the line of fusion and forms a continuous layer upon the 

 dorsum of the embryo, thus leaving the neural groove, now the neural tube, 

 extending the entire length of the embryo immediately beneath the ectoderm. 



The formation of the neural tube is a fundamental process, occurring 

 at an early period. It is obvious that any interference with its development 

 will be followed by serious defects in the nervous system and the structures 

 that immediately surround it. A most natural result of such interference would 

 be the failure of the two margins of the neural groove to unite, and it is not 

 improbable that the various forms of cranioschisis are the results of imperfect 

 or complete lack of closure of the cephalic end of the neural groove. Such 

 failure of the neural groove to close would leave the dorsum of the head region 

 open, so that not only the brain but also the cranial vault would be affected. 

 If the failure to close is complete, a high degree of acrania would result. In 

 case of partial closure some form of hemicrania might follow. 



Rachischisis, with partial or total absence of the spinal cord, may also be 

 attributed to defective closure of the neural tube, total rachischisis being due to 

 complete lack of closure, partial rachischisis to partial lack of closure. 



The origin of spina bifida has been a much discussed question. The earlier 

 view that the deformity was due to accumulation of fluid within the spinal canal 

 and rupture of the distended sac is now usually considered untenable. At the 

 present time it is generally agreed that spina bifida is closely related to defective 

 closure of the neural tube, although the exact nature of this relation is not 

 known. 



According to one investigator the defective fusion of the margins of the 

 neural groove is due to deficient growth of the blastoderm (von Reckling- 

 hausen). Another view is that the separation between the neural tube and 

 the adjacent ectoderm is incomplete (Torneux). Still another investigator 

 considers the defective development due to some primary defect in the germ 

 (Ziegler). Experimental studies on the frog's egg suggest to another observer 

 that spina bifida is caused by defective closure of the blastopore (Hertwig). 

 Recently it has become possible to produce spina bifida at will in some of the 

 lower Vertebrates (frog, Axolotl) by treating the developing eggs with a solution 

 of sodium chlorid (Hertwig). At the same time other defects in the nervous 



