TYPES OF VERTEBRATE AND INVERTEBRATE EYES 33 



for by the obliquity of the primary median antero-posterior axis of the two 

 heads. If these had been in the same plane and at right angles to the 

 primary dorso-ventral median planes of the two conjoined embryos, each 

 secondary face would have been complete and similar, as in the Janus- 

 headed monster shown in Fig. 24, A and B. In this example each 

 " secondary face " has two eyes, one of which belongs to the foetus 

 designated Y, the other to the twin foetus Z. 



Another type of double-headed monster — Duplicitas anterior (Dipro- 

 sopus) — in which cyclopia sometimes occurs is shown in Fig. 25. In this 

 form the two faces look forward in the same direction and the cyclops eye 



Fig. 25. — Diprosopus. Duplicitas anterior. 



Median eye formed by the union of the nasal half of the left eye of the right foetus, 

 with the nasal half of the right eye of the left foetus. 



(After Sbmmering, from Die Morphologie der Missbildungen : Schwalbe.) 



is bounded by the inner or nasal segments of four eyelids (two upper 

 fused together, and two lower). The eye itself is formed by the growth 

 in contact with each other of the inner or nasal halves of the opposed 

 organs, the temporal halves being suppressed. The auricles of the 

 external ears on the opposed sides are also suppressed, but a small opening 

 below the palpebral aperture and at the same level as the mouths represents 

 the common external auditory meatus. An X-ray photograph showed 

 two vertebral columns extending as far as the single pelvis, where fusion 

 took place in the sacral region. There was complete absence of the arms 

 and legs on the opposed sides. Notwithstanding the marked difference 

 3 



