THE EMBRYO. 1187 



follicles and ducts. The development of the former, however, remains imperfect, 

 except in the adult female. 



Development of the Limbs. The upper and lower limbs begin to project, as 

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

 These buds are formed by a projection of the somatopleure from the point where 

 the mesoblast splits into its parietal and visceral layers, just external to the verte- 

 bral somites, of which they may be regarded as lateral extensions. The division 

 of the terminal portion of the bud into lingers and toes is early indicated, and soon 

 a notch or constriction 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, of which the whole projection is at first composed, is differ- 

 entiated into muscle and cartilage, before the appearance of any internal clefts for 

 the joints between the chief bones. 



The muscles become visible about the seventh or eighth week. The voluntary 

 muscles are developed from the muscle-plates of the protovertebral somites, which 

 are at first segmentally arranged on either side of the rudimentary spine. Each 

 muscle-plate becomes differentiated into two parts, superficial and deep. The 

 former is termed the cutis plate, and from it the corium or true skin is developed, 

 while the latter becomes developed into longitudinal groups of muscle-fibres, ex- 

 tending forward into the neck and head region of the embryo and laterally to 

 enclose the cavities of the thorax arid abdomen. The muscles of the limbs are also 



Maxillary process. 

 Stomodseum. 



Mandibular arch. 



l^^li 



.^JHIlfeK 



Aortic bulb.-Tm~ 



Ventricle. 

 Vena cava superior. 



Umbilical vein.- 



FIG. 743. Heart of human embryo of about fifteen days. (Reconstruction by His.) 



formed from the same source, being produced by outgrowths from the protoverte- 

 bral somites in those situations where the limb buds appear. The involuntary 

 muscles are derived from the splanchnopleure mesoblast, and are therefore not 

 connected in any way with the protovertebral somites. 



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

 the development of the circulatory system, each in accordance with the manner in 

 which nourishment is provided for at different periods of the existence of the in- 

 dividual. In the first stage there is the vitelline circulation, during which nutri- 

 ment is extracted from the vitellus or contents of the yolk-sac. In the second 

 stage there is the placental circulation, during which nutrition is obtained by 

 means of the placenta from the blood of the mother. In the third stage there is 

 the complete circulation of the adult, commencing at birth, 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-sac. t consists of a 

 median tubular heart, from which two vessels (arteries) project anteriorly, 

 carry the blood to a plexus of capillaries spread over the vascular area, from whic 



