372 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



flexure in addition to those already described. The nature of this 

 flexure is shown in the upper portion of Fig. 177, A, which repre- 

 sents diagrammatically the conus of Zepidosiren as seen from the 

 ventral side. 



The extreme anterior end of the tube, being fixed firmly at its 

 exit from the pericardiac cavity, retains its primitive morphological 

 position : its originally dorsal side is actually dorsal. 



Traced backwards the tube is seen to become sharply bent upon 

 itself in a headward direction, in such a way that the side of the 

 tube which was originally on the left side comes to be ventral, as' is 

 indicated by the finely dotted line L in the figure. Tracing the 

 tube onwards a second sharp flexure is found and the tube resumes 

 its antero - posterior direction. This second flexure involves a 

 complete reversal of the tube. Its originally right-hand edge 

 (indicated by the coarsely dotted line R), which had come to be 

 dorsal as a result of the first flexure, is now ventral. 



The changes in the position of the tube caused by the two 

 flexures may be summed up by saying that the half of the tube 

 which was originally dorsal, and which remains dorsal at its 

 anterior or headward end, has come to be situated on the right side 

 at the posterior or ventricular end of the portion of the tube now 

 under consideration (conus). Similarly the half of the tube which 

 at the headward end is ventral, has come to be at the ventricular 

 end on the left side. 



The lower half of the diagram represents the portion of cardiac 

 tube which gives rise to the main part of the heart and it is to a 

 certain extent hypothetical, inasmuch as it does not rest on a 

 complete series of observations, but it is clear that the morpho- 

 logically right side of the cardiac tube, which is topographically 

 ventral in the middle part of the figure, has to get back to its 

 original right-hand position at the hinder end of the cardiac tube 

 (which like the front end is firmly fixed in position), and it is 

 reasonable to infer that the flexure of this portion, which gives rise 

 to the atrium and ventricle, would be found, were its unravelling 

 possible, to be symmetrical with the anterior flexure already dealt 

 with. 



It is interesting to take such a model as that represented in 

 Fig. 177 and subject the conus portion to a process of straightening 

 out — such as would happen in nature if the conus were to shrink 

 in length, its anterior and posterior ends remaining fixed. The 

 result is shown in Fig. 177, B. The conus assumes a twisting in a 

 right-handed spiral through three right angles. In the Amniota it 

 will be found that the representatives of the conus of the Lung-fish — 

 the roots of the great arteries, pulmonary and systemic — as they 

 pass head wards from the ventricular part of the heart, twist round 

 one another in just such a spiral. 



(2) As regards the increase in diameter of the cardiac tube, it is 

 characteristic that this does not take place equally throughout. At 



