126 DE VEL OPMENT. 



buds, from the anterior and posterior part of the embryo about the fourth week. 

 These buds are formed by a projection of the somatopleure (i. e. the outer layer 

 of the mesoblast and the epiblast), from the point where the mesoblast splits into 

 its parietal and visceral layers, just external to the vertebral somites, of which 

 they may be regarded as lateral extensions. The division of the terminal portion 

 of the bud into fingers and toes is early indicated, and soon a notch or constric- 

 tion marks the future separation of the hand or foot from the forearm or leg. 

 Next, a similar groove appears at the site of the elbow or knee. The indifferent 

 tissue or blastema, of which the whole projection is at first composed, is differen- 

 tiated into muscle and cartilage before the appearance of any internal cleft for 

 the joints between the chief bones. 



The muscles become visible about the seventh or eighth week. They are 

 derived from the protovertebral somites, and are consequently at first arranged in 

 segments, a condition which is retained by some of the deeper muscles of the 

 back and by the intercostal muscles. Fusion of successive segments takes place, 

 however, and further differentiation of the muscular sheet thus formed into a 

 varying number of muscular bundles brings about the adult condition. The 

 muscles of the limbs are produced from outgrowths from the protovertebral 

 somites in the regions in which the limb buds appear. 



Development of the Blood-vascular System. There are three distinct stages 

 in the development of the circulatory system before it arrives at its complete or 

 adult condition, in accordance with the manner in which nourishment is provided 

 for at different periods of the existence of the individual. In the first stage there 

 is the vitelline circulation, during which nutriment is extracted from the yolk or 

 contents of the vitelline membrane. In the second stage there is the placenta! 

 circulation, which commences after the formation of the placenta, and during 

 which nutrition is obtained by means of this organ from the blood of the mother. 

 In the third stage there is the complete circulation of the adult, commencing at 

 birth, and during which nutrition is provided for by the organs of the individual 

 itself. 



1. The vitelline circulation is carried on partly within the body of the embryo 

 and partly external to it in the vascular area of the yolk. It consists of a median 

 tubular heart, from which two vessels (arteries) project anteriorly. These carry 

 the blood to a plexus of capillaries spread over the area vasculosa, and also, though 

 to a less extent, in the body of the embryo. From this plexus the blood is returned 

 by two vessels (veins) which enter the heart posteriorly, and thus a complete cir- 

 culation is formed. 



In these vessels and the heart a fluid (blood} is contained, in which rudimentary 

 corpuscles are found. The mode of formation of these elementary parts will have 

 first to be considered. 



In mammalia the Heart is formed as a longitudinal fold of the splanchnopleure 

 on either side of the median line in front of the anterior extremity of the rudi- 

 mentary pharynx, at about the level of ^ the posterior primary cerebral vesicle, 

 the folds projecting dorsally into the coslom. The walls of the folds thicken and 

 present two distinct strata of cells ; the inner and thinner layer forms the endo- 

 cardium, the outer and thicker the muscular wall of the heart. In its very earliest 

 and primitive condition the heart consists, therefore, of a pair of tubes, one on 

 either side of the body. These, however, soon coalesce in the median line, and, 

 fusing together, form a single central tube. 1 Each of the two primary tubes 

 receives posteriorly a large vessel (a vein), and is prolonged anteriorly into 

 a second vessel (an artery). So that after fusion of the heart-tubes has taken 

 place, there is, in the primitive vitelline circulation, as above mentioned, a 

 single tubular heart, with two arteries proceeding from it and two veins empty- 

 ing themselves into it. The earliest vessels are also formed in the visceral layer 

 of the mesoblast. They are developed from that part of the mesoblast which sur- 

 rounds the portion of blastoderm which is occupied by the developing body of the 



1 In most fishes and in amphibia the heart originates as a single median tube. 



