533 



Specimen C (see Fig. 2). In this case there were two ap- 

 parently independent embryos which were very closely approximated 

 in their cranial portions but widely divergent at their caudal ends. 

 They were lying nearly transversely to the long axis of the egg on a 

 blastoderm which, although normal in outline, clearly showed duplicity 

 in the formation of the area pellucida and was surrounded by a single 

 sinus terminalis. It had been incubated for about 42 hours. Each 

 of the two appeared to be quite normal apart from a slight flexure 

 which was more marked in one and each possessed a double row of 

 mesoblastic somites 17 in number. Tlie hearts also were separate and 



/F.B. 



S. T. 



r-A.C. 



\~A.P. 



Figure 

 as before. 



Photographof Specimen C. A.C. Auditory Pit. Other letters 



not connected by any of the main vessels. Both the brains were nor- 

 mal and possessed well marked optic vesicles, indications also of the 

 primordia of the cranial nerves (VII and VIII) and (IX and X) and well 

 marked auditory pits. The heads were beginning to turn outwards 

 so that the two anterior ends were back to back and in front of each 

 was an amniotic fold already partly covering the front end. An 

 example somewhat similar to this only in an earlier stage of devel- 

 opment (i. e. with 7 mesoblastic somites) was described by W. B. Spencer 

 (12) but they appear to be uncommon. 



It is not intended here to discuss at any length the causes of 

 this duplicity a subject on which much has been written notably by 



