THE EMBRYO. 1165 



sinuses with their contained villi. Owing to the rapid thinning of the placenta at 

 the periphery of the disk, the decidual and chorionic surfaces come into contact. 



Separation of the Placenta. After the birth of the child the placenta and 

 the membranes (i. e., the amnion and chorion) are expelled as the after-birth, the 

 separation of the placenta from the uterine wall taking place through the basal 

 layer of the decidua and necessarily causing rupture of the uterine vessels. The 

 orifices of the torn vessels are, however, closed by the firm contraction of the 

 uterine muscular fibres, and this, together with the formation of a blood-clot over 

 the placental site, prevents post-partum hemorrhage. The epithelial lining of the 

 uterus is regenerated from the epithelium which lines the persistent portions of the 

 uterine glands in the basal layer of the decidua. 



The umbilical cord appears about the end of the fifth week after conception. 

 It consists of the coils of two arteries and a single vein, the umbilical arteries and 

 vein, united together by a gelatinous tissue (jelly of Wharton). There are origi- 

 nally two umbilical veins, but one of these vessels becomes obliterated, as do also 

 the two omphalo-mesenteric arteries and veins and the duct of the umbilical vesi- 

 cle, all of which are originally contained in the rudimentary cord. The umbilical 

 cord also contains the remains of the allantois, and is covered externally by a layer 

 of the amnion, reflected over it from the umbilicus. 



DEVELOPMENT OF THE EMBRYO. 



The further development of the embryo will, perhaps, be better understood if 

 we follow briefly the principal facts relating to the development of the chief parts 

 of which the body consists, viz., the spine, the cranium, the pharyngeal cavity, 

 mouth, etc., the nervous centres, the organs of the senses, the circulatory system, 

 the alimentary canal and its appendages, the organs of respiration, and the genito- 

 urinary organs. The reader is also referred to the chronological table of the devel- 

 opment of the foetus at the end of this section. 



Development of the Spine. The first steps in the formation of the spine have 

 already been traced, viz. : (1) The looping up of two longitudinal folds from the 

 cells of the epiblast in front of the primitive streak, to form the neural groove, and 

 the gradual growing together of the lamina? dorsales so as to convert the groove 

 into the neural canal. (2) The formation on the ventral aspect of this groove of a 

 continuous cellular cord, the notochord or chorda dorsalis (Fig. 700), which ex- 

 tends from the cephalic to the caudal extremity of the embryo, and lies in the 

 situation which is afterward occupied by the bodies of the vertebrae. (8) The 

 segmentation of the paraxial mesoblast on either side of the neural canal into a 

 number of quadrilateral masses, the protovertebrce or mesoblastie somites (Fig. 701). 

 The process of segmentation commences in the cervical region and proceeds suc- 

 cessively through the other regions of the spine until a number of segments are 

 formed, which correspond very closely to the number of the permanent vertebras. 

 Subsequently the protovertebral somites divide into two parts a ventral and a 

 dorsal. From the ventral division the vertebrae are formed ; the dorsal is termed 

 the muscle-plate, and from it the muscles of the back are developed. From the ven- 

 tral division of the protovertebral somites masses of cells are budded off, which 

 grow inward toward the middle line, those from opposite sides meeting and 

 enclosing the notochord and extending dorsally around the neural canal, which they 

 also envelop. Fusion of the ventral divisions also occurs in the antero-posterior 

 direction, so that all trace of their originally segmented condition is lost and 

 the notochord and spinal cord are surrounded by a continuous investment of 

 mesoblast, the membranous vertebral column. This investment also extends 

 forward and envelops the primitive brain, forming the membranous or 

 primordial cranium. From this investment the base of the skull, the ver- 

 tebrae and their ligaments, and the membranes of the cerebro-spinal nervous 

 system are developed. The future vertebrae make their first appearance about the 

 beginning of the second month in the form of two small masses of cartilage which 



