90 MR. NOEL TAYLER ON A UNIQUE GASE OP 



The condition of the anterior portion of the gut is of extreme 

 interest, and here, as in the case of the other organ-primordia, 

 deviation from the normal is very great. 



In the first place is to be noted the apparent absence of any 

 sign of an oral plate, the formation of which, in the absence of 

 any head-fold, is of course impossible. The gut is remarkable 

 by reason of its extension forward to the prechordal region (see 

 PI. I. fig. 7 B). The anterior extremity of the chorda comes into 

 i-elation with the dorsal wall of the gut, which latter then effects 

 contact with the well-developed infundibular downgrowth of the 

 Diencephalon (fig. 7 B, inf.dep.) ; this contact extends for some 

 distance, throughout which the mid-dorsal wall of the gut is 

 markedly thickened, showing some signs of cellular proliferation. 



The most surprising circumstance is, however, that in front of 

 this point, at which one would expect the gut to terminate, it 

 extends for a considerable distance ventrally to the Telencephalon 

 (PL III. fig. 4, E 2, and PI. I. fig. 7 B, Ant.ext.F.G.), from which 

 it is separated by a large vessel formed by the union of the two 

 apparently dorsal portions of the mandibular aortic arches. It 

 is only in the extreme anterior region of the Telencephalon 

 (PI. III. fig. 4, E 1) that the fore-gut has disappeared. 



d. Heart, vascular system, and ccelom. 



The Heart, as seen in the whole mount, is characterised by its 

 relative shortness and broadness (PI. I. fig. 3, Ht.). It presents an 

 outwardly-directed convexity on either side. The sections show 

 that the two endocardial tubes have completely failed to fuse, 

 and that they are, for the greater part of their extent, widely 

 sepai'ated. Only immediately in front of the anterior intestinal 

 portal do they even approximate in the mid-line, and even here 

 no fusion, seiisu stricto, has occurred. The right heart-tube is 

 somewhat shorter, and for the greater part of its extent its 

 diameter is considerably less than that of the left (PL III. fig. 5, 

 E 5, and PL II. fig. 5, E 6). 



It will be seen that the condition of the heart resembles that 

 which is sometimes described in an otherwise normal chick under 

 the name of " double-heart," and which was elucidated by Camille 

 Dareste (2), who showed it to result from the failure of the two 

 heart-tubes to fuse as normally occurs. 



Tracing the heart-tubes anteriorly they do not fuse beneath 

 the ventral wall of the gut to form the ventral aorta as in the 

 normal chick, but in the region of the infundibulum, where they 

 have become very small, they become continuous laterally with 

 two large vessels lying on either side of the upper surface of the 

 gut (as will be seen in the sequel, it is px-obable that these two 

 dorso-ventral vessels running on either side of the gut are not 

 morphologically the mandibular arches as one would at first sight 

 suppose). 



Anteriorly these two vessels (the right of which is the larger) 



