446 Mabel Bishop 



The embryo designated as Teras I represents another progressive 

 stage in the series under consideration. Aside from its value as a link 

 in this series, it has a still greater value in that it records a chapter in 

 the history of early embryonal development of a dicephalon and is, 

 therefore, extremely important in examining the theory that cosmobia 

 are at first perfectly orderly and symmetrical, and that the deviations 

 found in them are due to a lack of balance in later physiological activities. 

 Hence it has seemed best to reserve the study of this specimen for a 

 separate paper, owing to the technical difficulties in studying an embryo 

 of such size. I have noted it in the list of material, however, and 

 <iescribe it here in general in order to make the series as complete asi 

 possible. 



Teras I was received by Professor Wilder in an excellent state of 

 preservation so that he was able to make from it a set of beautiful serial 

 sections, which have been loaned to me for study. In consequence, 

 Plate II was made from sketches belonging to Professor Wilder, and the 

 description of the creature in toto has been made from his notes. 



Teras I corresponds to the specimen of human diprosopus tetroph- 

 thalmus represented by Forster [Tafel I, Fig. 4], Hirst and Piersol 

 [page 148, Fig. 74], and Ahlfeld [Tafel IX, Fig. 5]. It measured 

 22 mm. in length, was bilaterally symmetrical and well developed. Tlie 

 median eye was composed of two eyeballs in one socket; no median ear 

 was present. The two snouts were more separated than in the case of 

 Teras XII, thus defining the embryo as representing a later stage in the 

 cosmobiotic series — a stage between Teras XII and Teras XV. 



Concerning the history of Teras XV, I know little, except that it is 

 apparently a fully formed dicephalous lamb delivered at term, and that it 

 lived to be from three to four weeks old. Behind the region of doubling, 

 that is, the head region, the lamb presents externally a perfectly normal 

 appearance. The two heads are identical in contour, and of practically 

 the same size, their differences being extremely slight as the following 

 measurements will show: — Length of a straight line from left ear of 

 head A to extremity of muzzle is 10.5 cm. ; from the right ear of B to 

 same limit is 10 cm.; a similar line from the dorsal rim of the left eye 

 orbit through middle of eye to angle of jaw measures 6 cm. in both heads. 

 An inner lateral line across the muzzle posterior to the nostrils is 3.5 

 cm. long in A and 3.8 cm. in B. The distance between the tips of the 

 muzzles is 17.3 cm. 



The most noticeable difference between them is in the angle of 

 attachment of the two heads. Certainly one head, at least, suckled 



