1016 EMBRYOLOGY. 



of the vertebral type. The alliance of the embryonal and extra-embryonal 

 blastoderms has, at the commencement, a form that allows us to understand the 

 origin and mode of formation of the foetal annexes — umbilical vesicle, amnion, 

 chorion, aUantois. Before proceeding to the description of these organs, we will 

 examine precisely their origin. 



It has been already shown that the umbilical vesicle is nothing more than 

 the whole of the extra-embryonal splanchnopleure. The amnion is derived from 

 the extra-embryonal somatopleure ; from F to I this derivation may be followed. 

 Around the embryo the somatopleure undergoes a double inflection — one that 

 establishes the limits between its two embryonal and extra-embryonal parts, 

 and passes up towards the back of the embryo ; and another — C— which is 

 continued over the inner face of the vitelline membrane, and is named the fold ; 

 the amniotic layer comprised between the two inflections constitutes the amnion. 

 This membrane is made complete by the union of the amniotic folds (I) ; the 

 embryo is entirely enclosed in the amnion. The union of the amniotic folds 

 entails, at the same time as the occlusion of the amnion, the occlusion of the 

 remainder of the extra-embryonal somatopleure, which lines the whole of the 

 internal face of the vitelline membrane, and forms a continuous and independent 

 membrane — the serous layer, or, better, the chorion. 



The umbilical vesicle, the amnion, and the chorion, circumscribe a space 

 named the external calome, and communicating freely with the pleuro-peritoneal 

 cavity. 



In Figs. E and H, showing longitudinal sections, are seen the general 

 relations of all these parts. At al, Fig. Gr, is observed the origin of the aUantois, 

 which appears to be only a diverticulum of the last portions of the intestines. 

 The allantoid sac soon passes into the ccelome, and there develops in size and 

 shape according to the species of the animal. 



In the preceding chapter, we have seen by what evolutive modes the foetal 

 envelopes or membranes are derived from the intra-embryonal part of the somato- 

 pleure and splanchnopleure. These envelopes or membranes are represented in 

 the diagrammatic figure, 553. To them are joined the cord of vessels and the 

 capillaries that establish the relations between the mother and progeny. The 

 whole receive the name of annexes of the foetus. 



CHAPTER II. 

 THE PCETAL ENVELOPES OP SOLIPEDS. 



Preparation. — The dissection of the annexes of the foetus is a delicate operation ; but a 

 good idea of their arrangement may be obtained in several ways. 



1. The embryo should be first removed from the uterus. This is very easily effected in 

 Solipeds, as the adhesions between the placenta and the uterus are nearly always destroyed when 

 dissection is undertaken. With the foetus of Ruminants, it is necessary to open the uterus 

 from the cervix to the summits of the cornua, the incision passing between the cotyledons. la 

 order to accomplish this, the point of the branch of a pair of scissors is passed between the 

 chorion and uterus, in raising the wall of the latter. The cotyledons are then sought for. 

 With the foetus of the Cow, the chorion is carefully pulled from the uterine cotyledon in order 

 to separate the placentas ; with that of the Sheep and Goat, the cotyledons are pressed between 

 two fingers, in order to expel the chorial villi from their interior. 



