700 DEVELOPMENT OF THE F(ETAL MEMBRANES. 



wall of the trunk in two determinate places, whch are nearly the 

 same in all vertebrate animals, and receiving prolongations of the 

 33ones, muscles, nerves, and blood-vessels corresponding to a certain 

 number of the vertebral somatomes in the situation of the anterior 

 and posterior limbs respectively. 



2. EXTBA-EMBKYONIC PHENOMENA OF DEVELOPMENT OP THE OVUM. 



Foetal Menilbranes. — While the changes before described in the 

 central part of the blastoderm lead to the formation of the rudiments 

 of the embryo, there are simultaneously developed in its peripheral 

 parts, or extended into them from within, certain membranes which lie 

 external to the body of the embryo, but are for a time more or less 

 organically connected with it by the original continuity of the blasto- 

 dermic elements in which both sets of parts originate. 



Of these membranes, the yolk-sac exists in all vertebrate animals ; 

 the aniuion and allantois are common to birds and mammals, bub 

 are absent in amphibia and fishes ; and the chorion, in the sense in 

 which the name will be employed here, may be considered as peculiar 

 to mammals. 



Tlie Yolk-sac. — This name is given to an organised and vascular 

 covering formed by the extension of the layers of the blastoderm over 

 the surface of the yolk within the original vitelline membrane. In 

 human embryology it has also received the name of umbilical vesicle. 

 It consists originally of all the layers of the blastoderm, and in fishes 

 and amphibia retains these throughout the whole term of development "' ; 



Fig. 512. — Diagrammatic Sectio>'S of the Ovum in different stages of r)EVELOi> 



MENT TO SHOW THE PROGRESS OF FORMATION OF THE MEMBRANES (from Kollikcr). 



1. Ovum in which the choriou has begun to be formed, with the blastoderm and rudi- 

 ment of the embryo within. 2. Ovum in which the cephalic and caudal folds have con- 

 tracted the itmbilical ui)erture towards the yolk-sac, and the amniotic folds are turning 

 towards the dorsal aspect. 3. The amniotic folds being completed have met in the dorsal 

 region ; tiie umbilical opening is more contracted, and the allantois has begun to sprout. 

 4. The true amnion is detached from the reflected or false amnion which has disai3i3earecl 

 or combined with the chorion ; the cavity of the amnion is more distended ; the yolk-sac 

 is now pediculated, the allantois projects into the space between amnion, choriou, and 

 yolk-sac, and the villi of the chorion begin to ramify. 5. The ovum when it has become 

 embedded in the uterine decidua ; the yolk-sac (umbilical vesicle) is now connected to 

 the fcetus by a long duct, the amnion is increased in volume ; the allantois remains ouly 

 as a pediculated vesicle towards the attachment of the short umbilical cord to the part 

 of the chorion where the placenta is about to be formed. The vascular layer of the 

 allantois has now combined with the chorion, the villi of wliieh have undergone further 

 development. 



d, vitelline membrane or ijrimitive chorion ; d', commencing villi of the chorion ; sp, 

 epiblast ; sz, villi of the chorion more advanced ; ch, permanent chorion with which 

 the vascular layer of the allantois is combined : cJi, z, true vascular villi of the chorion ; 

 am, amnion ; uh, its cavity ; Ics, cephalic fold ; ss, caudal fold of the amnion ; a, 

 the ejubryonal rudiment in the epiblast ; m, that in the hypoblast or mesoblast ; st, 

 margin of the vascular area in its early stages : dd, hypoblast ; kli, hollow of the vesicular 

 blastoderm, becoming afterwards d», the hollow of the yolk-sac ; dg, ductus vitello- 

 mtestinalis ; al, allantois ; e, embryo ; v, original space between amnion and chorion ; 

 -vl, wall of the thorax in the region of the heart ; M, pericardial cavity. 



The batrachia seem to be an exception to this statement, but only in consequence of 

 tlie yolk sac being so extremely limited that it merges in the intestine itself. The yolk- 

 sac and primitive intestine are in fact combined togethei", there being no umbilical con- 

 striction between them. 



