414 AN INTRODUCTION TO ZOOLOGY 



grow inwards towards the middle line between the ecto- and entoderm 

 layers, thus making the gut wall at this point very deep. The 

 second vessels to appear are a pair of ventral aortse, beneath and 

 ventral to the fore gut, and these run backwards and pass out behind 

 the opening leading into the fore gut, i.e. the anterior intestinal 

 portal, into the vascular network as the vitelline veins. These two 

 thin tubes run along the inner side of the vertical portion of meso- 

 derm of the mesial walls of the amnio-cardiac vesicles, whose walls 

 are thickened in this region to form the myocardium, which is 

 destined to give rise to the main mass of the muscles of the heart. 

 Very soon after, the vesicles fuse in the middle line to form a median 

 space, the future pericardium, and the two endothelial tubes also 

 fuse over part of their length to form a single vessel, the endo- 

 cardium, which provides the lining of the various chambers of the 

 heart. The fusion of the vesicles forces the anterior intestinal portal 

 backwards. The heart rudiment then comes to be a median tube 

 continuous with two posterior limbs diverging as the vitelline veins, 

 and passing forwards into the two ventral aortae, which communicate 

 around the front end of the fore gut with the dorsal aortae by con- 

 necting trunks termed the first or mandibular arches. 



The heart tube itself increases in length, but as its anterior and 

 posterior ends are relatively fixed, the enlargement results in a 

 bulging out to the right side, and so allows two divisions to be 

 recognised, an anterior ventricular portion and a posterior atrial 

 portion. To the hinder end a further region, the sinus venosus, is 

 added by a fusion of a part of the vitelline veins. Further growth 

 leads to a still more pronounced bending, and the ventricular region, 

 the anterior end of which is by this time differentiated to form a 

 bulbus arteriosus, gradually moves down ventral to the atrial region, 

 ultimately coming to lie behind it. Next the atrium is divided into 

 two by the growth of the inter-atrial septum, and a union and en- 

 largement of the proximal ends of the vitelline veins forms the sinus 

 venosus. Finally, the bulbar portion becomes absorbed into the 

 right side of the ventricle, so that with the division of the ventricle 

 into two, as a result of the development of the interventricular septum, 

 the bulbus arteriosus comes to be incorporated with the right 

 ventricle only. 



As noted previously, at the thirtieth hour the dorsal and 

 ventral aortae are united by but a single arch on each side, but during 

 the second day, firstly, the second or hyoid arch is formed and then 

 a third arch. By the end of the third day a fourth arch has been 

 completed, and the fifth and sixth arches are established during the 

 fourth and fifth days. Of these arches the fourth and sixth are most 

 strongly marked, while, the fifth is only transitory and incomplete. 



