ENCEPHALOCELES AND OTHER ABNORMALITIES. 91 



two-tailed. He referred to them as spina bifida and gave a clear explanation of 

 the mechanics of the process producing them. However, the relation of this type 

 of spina bifida to the more ordinary condition of a single open neural canal is not 

 altogether plain, and his suggestion that "imperfect oxidation" causes spina bifida 

 does not further clarify the question. 



The earliest picture of the subcutaneous type of spina bifida with which 

 we are familiar is that encountered in embryos around the 10 mm. stage of devel- 

 opment, in which the neural tube is everywhere closed, showing, however, a 

 greater or less area of enlargement. Such a state has not as yet been experimentally 

 produced. Several explanations have been advanced to account for it, none of 

 which are satisfying, nor substantiated by evidence. One suggestion is that the 

 enlargement of the neural tube is due to the fact that dorsally it remains 

 attached to the ectoderm (non-separation of the membrana reuniens). On the 

 other hand, it has been suggested that the neural tube becomes enlarged because 

 of increased pressure from the contained fluid. In this connection it would seem 

 that when the affected areas are limited in extent they are in some way connected 

 with the curvatures of the body, since such areas usually occur in the neck or 

 sacral region, where, in the embryo, the curves are most pronounced. The process 

 is supposed to be one of subsequent pressure of the hydromyelia on the sur- 

 rounding parts, thus inhibiting the development of cartilage and bone. With our 

 advance in knowledge regarding the circulation of the cerebro-spinal fluid, some 

 of the most puzzling features presented by subcutaneous spina bifida will prob- 

 ably be satisfactorily explained. The work of Weed on the normal cerebro-spinal 

 fluid circulation is most helpful, supplying as it does for the first time an adequately 

 correlated picture of the formation and extension of the cerebro-spinal fluid with 

 the differentiation of the perimedullary mesenchyme to form the meninges. In 

 the meantime, any discussion concerning the etiology of subcutaneous spina bifida 

 is entirely theoretical. Suggestions have been made that it may arise directly 

 from the open spina bifida form; again, that it may be the result of some entirely 

 different process, or that both forms may be produced by the same pathological 

 agent acting at a different stage of development. While it has been generally 

 assumed that the open and subcutaneous forms of spina bifida are related, this 

 has never been proved. A more definite picture of each process must be obtained 

 before we can know the nature of the relation, or whether there is such a relation, 

 existing between the two. The literature on the subject gives the impression that, 

 although sound facts and more or less sound theories regarding spina bifida have 

 multiplied, there is much that is not clear and that must be understood before we 

 can have a comprehensive insight into the processes producing it. That this 

 information may be gained through a closer embryological study seems probable. 



The study of the specimen dealt with in this paper has been made chiefly along 

 morphological lines. Only a meager clinical history regarding it was obtainable; 

 the child was illegitimate, was born spontaneously at full term, and lived only a 

 few hours. Its external form is shown in figures 1, 2, and 3, and various measure- 

 ments are given in table 2. Externally, the most marked abnormality is the 



