ASYMMETRICAL DUPLICITY IN THE CHICK, 91 



pass round the median infundibular-gut junction (E 3) and fuse 

 to form a large median aortic space lying between the Telen- 

 cephalon and the prechordal prolongation of the gut (PL III. 

 fig. 4, E 2, Ant.aor.Sp.). Posteriorly the two vessels (PI. III. 

 fig. 4, E 3, R.Lat.aor.Sp., L.Lat.aor.sp.) become continuous with 

 the dorsal aortse (E 5-E 6, Dor.Aor.), which pass back on either 

 side of the chorda in the normal manner. 



The myocardium is characterised by its width ; in front of the 

 anterior intestinal portal there is a ventral myocardial septum. 



With respect to the pleuro- pericardial coelom, this is remark- 

 able for its extreme forward extension immediately beneath the 

 embryo, the lateral amniocardiac vesicles opening into each other 

 and forming a median coelomic space which extends in front of 

 the anterior limit of the embi-yonal formation proper. This 

 forward extension is permitted by the absence of the head-fold 

 (PI. Ill, fig. 4, E 1 and E 2, Ant.p.p.c; text-fig. 2, Ant.p.p.c), 

 and in tlie sequel it will be seen to be a point of considerable 

 morphogenetic significance. 



Text-figure 2. 



">*» ..» -siji^ 



Transverse section through the extreme anterior region of embryonal 

 formation a. 



For explanation of letters see p. 109. 



D. Summary of the principal morphological abnormalities of the 

 greater Embryonal Fomnation (Rudiment a). 



A. General proportions of anterior region and its relations to 

 the extra-embryonal portions of the blastoderm. 

 I. Absence of an anterior amniotic fold. 

 II. Absence of a head-fold. 

 III. Absence of cranial flexure. 



lY. Abbreviation of head proper as indicated by the distance 

 between the anterior end and the plane of the auditory 

 vesicles. 



