GENERAL EMBRYOLOGY 1629 



villi pass through it, but have no connection with it. Subsequently 

 its cells become grouped into patches, and finally disappear. 



The superficial layer becomes converted into a tissue, composed 

 of nucleated protoplasm, but without any recognisable cells. This 

 layer is known as the syncitial layer. It is also spoken of as the 

 syncitium, being so named because it has no recognisable cells. In 

 the case of the fixation — or root — vWh, this s\Ticitium accompanies 

 them in the form of fihform processes, as they penetrate into the 

 uterine mucosa. 



The villi of the chorion frondosiim, as they project from the 

 surface, are separated by intervals, which constitute the maternal 

 blood-spaces, -sinuses, or -lacunae. These intervals are also known 

 as the intervillous spaces, and those villi which are free project into 

 them. These spaces are lined with sjTicitium, which tissue conse- 

 quently covers the surface of the chorion frondosum between the 

 villi, as weU as the villi themselves. 



The Allantois. 



The allantois extends as a hollow tubular diverticulum from the 

 dorsal waU of the yolk-sac to the inner surface of the chorion fron- 

 dosum. The hind-gut has not been differentiated at this stage, but 

 when this takes place the allantois becomes connected NN-ith the 

 ventral waU of the hind-gut. At this point it is placed on the 

 cephalic side of the cloacal membrane. It soon acquires an 

 important relation to the cloaca, this latter being a blind chamber 

 into which the hind-gut and urogenital ducts open. The allantois 

 becomes connected with the ventral wall of the ventral or uro- 

 genital compartment of the cloaca, the dorsal compartment per- 

 taining to the rectum. 



The aUantois consists of splanchnopleure (entoderm and 

 splanchnic, or visceral mesoderm), which is continuous with the 

 splanchnopleure of the wall of the gut. It soon elongates towards 

 the umbilical orifice, through which it leaves the body of the 

 embryo, lying on the caudal side of the vitelline duct. At this open- 

 ing the allantoic diverticulum enters the allantoic, or body-stalk. 

 This stalk traverses the chorionic cavity*, and ends by blending with 

 the inner surface of the chorion frondosum, which gives rise to the 

 foetal part of the placenta. The allantois consequently establishes 

 an important connection between the body of the embryo and the 

 placenta. 



In order to understand the fimctions of the allantois it may be 

 divided into two parts — ^intra-embryonic and extra-embryonic. 

 The intra-emhryonic part is distinguished as the allantoic diverti- 

 culum, and the extra-embryonic part is known as the allantoic, or 

 body-stalk. The allantoic diverticulum has just been described. 



The allantoic stalk extends from the umbilical orifice to the 

 placental area of the chorion. As has been incidentally stated, the 

 amnion and chorion do not become completely separated at the 



