MAM MALI AN EM BRYOLOGY 



631 



The decidua basalis forms what is called the maternal placenta and 

 the chorion frondosum the foetal placenta. 



The decidual membranes and 

 their attachments form the after- 

 birth. This afterbirth consists of 

 amnion, chorion, decidua vera, 

 placenta, and a part of the decidua 

 basalis. 



THE UMBILICAL CORD 



As the body-stalk becomes 

 longer and longer, finally reaching a 

 length of some fifty centimeters, 

 there must be some circulatory con- 

 nection between the embryo and the 

 chorion frondosum. This connec- 

 tion is brought about by the development of four blood vessels, two 

 veins and two arteries, known as the umbilical vessels or allantoic vessels 

 (Fig. 363). The two veins push their way into the embryo to open into 

 the heart. The arteries likewise grow in the same direction as do the 

 veins, but connect with the dorsal aorta. Their distal ends extend 



Fig. 362. 



Human Embryo. Age seven weeks. 

 (From Kollmann. ) cf, chorion frondosum. cl, 

 chorion laeve. 



Fig. 363. 



1 to 6, Diagrams representing six stages in the development of the foetal 

 membranes in a mammal. 



The ectoderm is indicated by solid black lines ; the entoderm by broken lines ; 

 the mesoderm by dotted lines and areas. (After Kolliker.) 



7, Diagram of nurture of young through embryonic membranes, g, gill circula- 

 tion of embryo ; h, heart ; i, dorsal aorta ; j, postcava ; k, allantoic artery ; 

 i, allantoic vein ; m, indicating the course of the blood of the mother, parallel 

 to n; n, that of the embryo; w, wall of uterus. (After Needham.) 



