NERVOUS SYSTEM OF A TWO-HEADED PIG EMBRYO 381 



death by accidental injury in September. Had he followed this 

 by a gross and microscopical study of the nervous system, its 

 scientific value would have been manifestly increased. 



I believe the time is not yet ripe for a final valuation of tera- 

 tology, since a more complete study of the internal anatomy of 

 terata after birth or hatching is needed, preceded in so far as 

 possible by careful observations of their behavior and physiology, 

 for external forms shed little light upon structures within, and 

 in almost every case of symmetrical double monsters, at least, 

 the duplicity is greater internally than externally. Detailed 

 studies of each system through a series of developmental stages 

 in embryos and foetuses of the same type of abnormality, fol- 

 lowed by comparative studies of the same stages in several types, 

 are of especial value, since the modifying influences undoubtedly 

 occur in embryonic life. And if such embryos occurring free in 

 nature are of true scientific value they should serve as a control 

 for experimental teratology and be capable of correlation not 

 only with this aspect of the teratogenetic problem, but also with 

 the whole scheme of the mechanics of normal development. 

 Until a large body of such data on free-in-nature embryos has 

 been recorded, I believe that the final evaluation of teratological 

 studies, especially of teratembryolog^^ cannot be given. 



Since intensive studies of the central nervous system have been 

 especially neglected, the present investigation places on record 

 the data gathered from a detailed study of this system in a young 

 mammalian embryo in accordance with the programme sug- 

 gested. No attempt has been made to add another review of the 

 voluminous literature on teratology, or to enter into a discussion 

 of the causation of abnormalities. 



DESCRIPTION OF THE MATERIAL 



The specimen studied is a two-headed pig embryo, 22 mm. in 

 length, bilaterally symmetrical and well developed. Before 

 being loaned to me for study, it had been cut into an almost per- 

 fect series of cross-sections, each 20 /i in thickness, and stained 

 with Delafield's hematoxylin and eosin. As I had not seen the 

 specimen in toto, a wax-plate model of the embryo rostral to the 



