64 THE SECOND DAY. [CHAP. 



embryo. The anterior extremity of the heart is connected 

 with the two aortse. 



The muscular portion of the walls of the heart are derived in the chick (as 

 in all other vertebrates in which the point has been worked out) from the 

 mesoblast of the splanchnopleure. 



Although thus much may be asserted with tolerable certainty for all verte- 

 brates; yet the exact mode of development appears, according to our present 

 knowledge, to be very different in different cases ; and it seems probable that 

 these differences are in part the result of variations in the mode of formation 

 and time of closure of the alimentary canal. 



In the chick the investigation of the earlier stages of the heart is beset with 

 considerable difficulties; and accordingly various inquirers have arrived at very 

 different results, though the majority are agreed as to its formation from the 

 mesoblast of the splanchnopleure. Exact information concerning the epithe- 

 lium lining the heart may be said to be almost completely wanting. 



Von Baer described the heart as consisting in its earliest stage of two solid 

 aggregations of the mesoblast cells of the splanchnopleure, converging in front 

 at the end of the foregut where they are loosely united together by a thin band, 

 but diverging behind along the diverging folds of the splanchnopleure. As the 

 foregut lengthens, the two masses coalesce more and more completely in front, 

 until the whole structure assumes the shape of a fusiform mass, attached to the 

 under wall of the foregut, with prolongations stretching like the limbs of 

 an inverted & along the folds of the splanchnopleure on either side. A t first 

 solid throughout, the ^-shape mass subsequently becomes hollowed out and 

 filled with fluid by the solution of its central cells. 



The account given by Eemak (EntwicTcelung der Wirbelthiere, 1855) is some- 

 what similar. 



According to His the heart is formed by the separation of a layer of the 

 splanchnopleure and its coalescence with a similar layer from the somatopleure. 

 It is therefore from the beginning hollow. Its cavity is also from the be- 

 ginning continuous with the canals of the aortae and omphalo-mesaraic veins, 

 the roots of which are formed in a precisely similar manner as itself. It is 

 through these that the epithelial (endothelial) elements, derived from the white 

 yolk, make their way into the heart to form its epithelial (endothelial) lining. 



According to Afanassieff (Bull. Acad. St. Petersbourg., Tom. xm. 1869, 

 pp. 221 335) the heart is formed by the longitudinal separation of a thick 

 layer of the mesoblast of the splanchnopleure along the under wall of the fore- 

 gut. At either side, so much of the mesoblast is detached, that only a single 

 layer of cells (seen spindle-shaped in a transverse section of the embryo) remains 

 united with the hypoblast to form the wall of the gut. Along the middle line 

 the separation is not complete, the detached layer of mesoblast being here still 

 connected with the wall of the gut by a few cells. The single layer of spindle- 

 shaped cells breaks loose in turn from the wall of the gut on either side of the 

 mill die line, but still remains attached along the middle line itself. We have 

 thus in transverse section a thinner and a thicker layer of mesoblast, hanging 

 down in a double festoon from the (hypoblastic) under-wall of the gut. Both 

 layers become more and more separated from the gut, and bulge out into the 

 pleuroperitoneal space, thus creating between themselves and the gut a cavity, 

 which is at first double, but, by the disappearance of the cells along the middle 

 line, subsequently becomes single. This is the cavity of the heart, the thick 

 layer representing its muscular walls, and the thin its epithelial lining. The 

 two ends are open, the hinder end being connected with the omphalo-mesaraic 

 veins, and the front with the aortse. At first the heart is not a tube with 



