96 MR. NOEL TAYLEE ON A UNIQUE CASE OF 



Congestion has taken place on this side, as is evinced by the 

 greater size of the i-ight niandibular arch. The greater resistance 

 to the tiovv of tlie blood on the left side, owing to the smaller size 

 of the vessels, has led to a congestion in the left ventral aorta and 

 heart-tube, hence its greater diameter. 



3. The miterior portions of the fore-gut and vascular systetn. 



There yet remain for consideration the anterior portion of the 

 fore-gut and its relations to the central nervous system. 



Firstly, with reference to the absence of an oral plate, a glance 

 at the figure of the longitudinal section (PI. I. fig. 7 B) will make 

 quite clear the impossibility of a fusion between the anterior gut 

 entoderm and adjacent ectoderm such as normally occurs, since 

 liere the pleui'O-pericardial cavities, which normally fuse in the 

 mid-line posteriorly to the oi-al plate (fig. 7 A, p.p.c), in this 

 case extend to the anterior end of the fore-gut and to some con- 

 siderable distance in front of it (PI. III. fig. 4, E 1-E 4, PI. I. 7 B, 

 p.p.c), indeed for some distance in fi'ont of the anterior end of the 

 embryo itself : this is a definite median coelom, formed jjresumably 

 by the fusion in the mid-line of the lateral ccelomic cavities. 



There is, indeed, no sign of any thickening of the floor of the 

 fore-gut in the region where one would expect fusion between 

 entoderm and ectoderm, were it possible to take place. 



]t will be remembered, however, that there is a very marked 

 tliickening and proliferation of the median gut-roof adjacent to 

 tlie infundibular region of the fore-brain (PI. III. fig. 4, E 3, E 4, 

 PI. I. 7 B), and if this entodermal thickening be regarded as the 

 protochordal plate complex, we can, I tliink, consistently inter- 

 pret the condition of the fore gut and its relations to nervous 

 and vascular systems with leference to the normal morphological 

 relations of these parts. 



In PI. I. fig. 7 B is given a semi-diagrammatic reconstruction 

 of a median longitudinal section of the major embryonal formation 

 of Blastoderm E alongside, for the purpose of comparison, of a 

 similar sagittal section of the anterior region of a normal embryo 

 of about the same age (fig. 7 A). 



It will be I'ecollected that in the normal chick the entoderm of 

 the roof of the extreme anterior end of the fore-gut is some- 

 what thickened in the vicinity of the termination of the chorda 

 and the floor of the diencephalon, a slight diverticulum in this 

 region forming the so-called prechordal gut or Seessel's pocket, 

 while an entodermal tliickening situated ventrally comes into 

 relation with the ectoderm to form the oral plate. The depres- 

 sion of the oral plate beneath the ventral surface of the head is 

 increased as development progi-esses by the growth of the brain 

 and the occurrence of the cranial flexures. The two lateral com- 

 ponents of the mandibular aortic arch meet in the mid-line just 

 behind the posterior limit of the oi-al plate to form the median 

 ventral aorta (fig. 7 A. X'). 



