Asymmetrical duplicity in the chick. 89 



the head, as defined by the distance between its anterioi' end and 

 tlie auditory vesicles, is barely '2 mm, less than normal. 



It is therefore clear that the abbreviation of the head of 

 Rudiment o is largely apparent, i-esulting from the abnormally 

 anterior position of the intestinal portal ; but is in part real, 

 since the head, as more strictly defined by the position of the 

 auditory vesicles, measures "2 mm, less than normal. 



b. The nervous system. 



The nervous system is, posteriorly to the auditory vesicles, 

 quite normal, its development only becoming abnormal in the 

 anterior region. 



All the chief divisions of the brain are clearly recognisable 

 (PI. I. fig. 3 ; Pis. II. & III. figs. 4, 5), and the five neuromeres 

 of the Hind-Brain are quite distinct. Cranial flexure being 

 absent, the neural axis is, of course, straight. 



The degree of abnormality increases as we pass postero- 

 anteriorly and is greatest in the Fore-Brain. This abnormality 

 is of two kinds : firstly, a general under-development of the Fore- 

 Brain, and, to a less extent, of the Mid-Brain ; secondly, a 

 definite bilateral asymmetry of the Diencephalon, evident parti- 

 cularly in the optic evaginations. 



The Mesencephalon appears somewhat compi-essed in the 

 sagittal plane. The Telencephalon is very small and its cavity 

 relatively minute. (PI. I. fig. 3 and PI. II [. fig. 4, E 1 and E 2, 

 Tel.). 



The asymmetry of the Diencephalon was a striking feature of 

 the whole mount (PI. I. fig. 3, Dlen.) which gave the impression 

 that the right optic evagination is entirely Avauting, the left being 

 i-elatively well developed and almost making contact with the 

 ectoderm. Reference to the sections through this region (Pi. III. 

 fig. 4, E 3 and E 4) shows that the cavity of the Diencephnlon is 

 extremely irregular in shape. The left optic evagination is well 

 marked (l.op.). The Diencephalic wall in the corresponding 

 position on tlie right side shows no such definite evagination, 

 though dorso-laterally and ventro-laterally it is bent outwards 

 somewhat suggesting commencing evaginations. The more dorsal 

 outbulging, which is the larger of the two, would seem to represent 

 the right optic vesicle, but if so it is very much less developed 

 than the left, the brain in this region being markedly asymmetrical 

 (fig. 3). A median ventral downgrowth of the diencephalic floor 

 is well marked even in the whole mount (fig. 3, inf.dep.) ; in trans- 

 verse section (fig. 4, E 4, inf.dep.) it is seen to approximate to a 

 median dorsal thickening of the gut-entoderm. It is probably 

 infundibular in nature. Further anteriorly the floor of the 

 diencephalon becomes very thick (PI. III. fig. 4, E 3). 



c. The fore- giU. 



Owing to the anterior position of the intestinal portal the fore- 

 gut is relatively very short. Throughout its length it is dorso- 

 ventrally compressed. The hyoid pouch almost makes contact 

 with the ectoderm (PI. ]I. i\c;^r^, E 7, l.Ih/.P.). 



