370 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



partially broken down walls, and the ordinary intercellular spaces of 

 syncytial embryonic connective tissue. 



GENERAL CONSIDERATIONS REGARDING THE MORPHOLOGY OF THE 

 VERTEBRATE HEART. It may be regarded as a primitive characteristic 

 of blood-vessels that their walls are contractile, peristaltic waves of 

 contraction serving to propel the blood in their cavities. It is 

 usually the case however in the more complex animals that this 

 contractility becomes concentrated in one or more localized portions 

 of the vessels, known as hearts, in which the vessel becomes much 

 enlarged and its muscular coating thickened and rhythmically con- 

 tractile. 



In the craniate Vertebrates there is one heart present and it 

 represents an enlarged portion of the ventral vessel in the region 

 immediately behind the gills. During ontogeny the heart still 

 repeats the archaic evolutionary phase in which it was tubular in 

 character. As development goes on the primitive heart, or cardiac 

 tube, shows rapid increase in size within the perieardiac chamber of 

 the coelome in which it lies. This chamber is relatively small in 

 size and in the lower, fish-like, Vertebrates is bounded by rigid 

 unyielding walls. The confined nature of this space in which the 

 heart has been evolved has, by imposing restrictions upon it during 

 its increase in size, exercised a profound influence upon the modelling 

 of the vertebrate heart. It is therefore desirable to have a clear idea 

 of the general relations of the heart to the perieardiac cavity, during 

 its increase in size, before attempting to study its development in 

 detail in the various groups of Vertebrates. 



The portion of vessel originally included between the anterior and 

 posterior limits of the perieardiac cavity will be referred to here as 

 the primitive heart or cardiac tube. As development proceeds 

 the increase in size of the primitive heart reveals itself in (1) increase 

 in length and (2) increase in diameter. 



(1) As regards the former, the cardiac tube is at its posterior 

 and anterior ends where it enters and leaves the perieardiac cavity 

 respectively firmly embedded in the tissues of the perieardiac wall. 

 These ends being consequently in the lower, fish-like, vertebrates 

 rigidly fixed in position, it has of necessity come about that the 

 cardiac tube, while in the course of evolution it has increased in 

 length, has lost its original straight form and has been thrown into a 

 system of bends or kinks which have had an important influence 

 upon the structure of the fully evolved heart. This hendinu process 

 is repeated, though with obscuring of some of its detail, during onto- 

 geny and it is an interesting morphological problem to endeavour to 

 unravel the details of the process from the data of comparat ive 

 anatomy and emhrvl' 



App.in-nlly the primary llexure of tin- cardiac tuhe is represented 

 by a simple loop or huliiin^ toward- iln- ri-lit side of the liody. which 

 il.le in the eiidu Lebratc during early stages of 



heart development. 



