94 MR. NOEL TAYLER ON A UNIQUE CASE OF 



where, according to Miss Parker, the initiation of head-fold 

 formation is due to the forward growth of the brain-plate, whilst 

 the expansion of the pericardium is the main factor responsible 

 for the extension of the fore-gut. 



A feature which is of undoubtedly great interest is that in an 

 embryo in which thei'e can have been very little active forward 

 growth (i. e., in contra-distinction to general or interstitial 

 growth) of the medullai-y plate, relatively to the line separating 

 embryonal and extra-embryonal areas, which notably Robinson 

 considered as the main factor concerned in fore-gut formation, 

 there should be present a well-marked gut extending anteriorly 

 \(p to the anterior limit of the Telencephalon. 



That a true head-fold could never have been present is fairly 

 evident from the sections (PI, III. fig. 4, El, E 2, E 3), and 

 accordingly it may be concluded that the second factor (pleui-o- 

 pericardial ccelomic expansion) must have been that which was 

 mainly if not solely operative in the formation of the entire 

 extent of the fore-gut. This is borne out by the presence of the 

 pleuro-pericardial coelom, separating somatopleure and splanchno- 

 pleure in the extreme anterior region of the embryo, in front 

 even of the anterior limit of the gut itself (fig. 4, E 1, and text- 

 fig. 2), a condition which is not found, of course, in the normal 

 chick embryo. It is of importance, too, to note that the slight 

 inturning of the ectoderm beneath the extieme tip of the embryo 

 does not extend to anywhere near the anterior limit of the gut 

 (fig. 4, E 1 & E 2 ; PI. I. fig. 7 B). 



It is probable, therefore, that in this embryo even the initiation 

 of fore-gut formation was due to the formation of the pleuro- 

 pericardial coelom. There must have been an extension in 

 towards the middle line, and a svibsequent union of the amnio- 

 cardiac vesicles, thus producing a condition somewhat similar to 

 the pleuro-pericardial canal of mammals. Expansion of the 

 pleuro-pericardial cavity so formed in the posterior direction 

 would result in the formation of a splanchnopleuric or cardiac 

 fold and a fore-gut. 



Now it will be recollected that the apparent length of the 

 fore-gut as determined by the position of the anterior intestinal 

 portal is 1*2 mm., as compared with a normal length of about 

 2"1 mm. This seems to indicate that while in normal morpho- 

 genesis the active forward growth of the brain plays a part in 

 the increase in length of the fore-gut, the absence of this factor 

 in specimen E must have influenced unfavourably the action of 

 pleuropericardial expansion, resulting in an unduly slow pro- 

 gression backwards of the cardiac fold, and an abnormally short 

 fore-gut, which is therefore a secondarily induced condition 

 resulting from the primary abnormality of the medullaiy plate. 



2. The condition of the heart- primordia. 



The earlier processes in the development of the heart stand 

 in intimate causal relationship to fore-gut formation, and one 



