vi PREGNANCY PARTURITION PUERPERIUM 233 



endodermic layer, forms the lower vesicle, adhering in great part 

 to the amniotic vesicle and also to the internal surface of the 

 chorion by means of the abdominal peduncle. 



The allantois, which is already well marked, although a 

 caudal intestine is not yet distinguishable, is never free in the 

 para-embryonic body-cavity ; it always passes inside the abdo- 

 minal peduncle. 



The chorion, which is derived from the original peripheral layer 

 of the blastocyst, is already provided with villi. Its large cavity 

 (cavity of the chorion or outer coelom or para- or extra-embryonic 

 cavity) contains the magma reticularis of Velpeau, which also 

 surrounds the embryonic formations. 



The successive transformations undergone by the membranes 

 of the human ovum are easily understood. The allantois, which 

 also acts as a urinary sac, extends rapidly over the whole of that 

 region of the chorion in which the discoidal placenta is being 

 formed. The essential part of this process pertains, not to the 

 epithelium of the allantois (endoderm), but to the blood-vessels 

 which it brings from the body of the embryo to the chorionic 

 villi. Its peduncle and its vessels, which in the first stages of 

 embryonic life are contained in the abdominal peduncle, are seen 

 later on to form part of the elements constituting the umbilical 

 cord. At birth the extra-embryonic portion of this cord is lost, 

 while the intra-embryonic portion continues to be of use as the 

 urachus and urinary bladder. 



We have already spoken of the transformations of the 

 chorion. 



The umbilical vesicle or vitelline sac, which originally rested 

 on a large base after the manner of a sessile tumour on the ventral 

 face of the body of the embryo, gradually becomes pedunculated 

 (Figs. 96 and 97). There is a period of embryonic life in which 

 this membrane lies unattached in the magma reticularis of the 

 external coelom. Its peduncle or omphalo-mesenteric canal is 

 later included in the umbilical cord, its swollen extremity lying 

 under the amnion a little to one side of the point of union of the 

 umbilical cord and the placenta ; at birth it disappears with the 

 after-birth. It is but seldom that in the intestine of the adult 

 man a remnant of the omphalo-mesenteric canal is found in the 

 form of the diverticulum of MecJcel in the lower part of the ileum, 

 a little above the ileo-caecal valve. 



The changes undergone by the amnion must be specially 

 studied with reference to the formation of the umbilical cord 

 which is first indicated by the abdominal peduncle. This peduncle, 

 when the body of the embryo is already formed (fifteen to eighteen 

 days), is situated on the dorso-caudal side of the embryonic forma- 

 tion and the amnion (Fig. 96). Up to this period the abdominal 

 peduncle contains only the allantoid canal and the allantoid or 



