304 COMPARATIVE ANATOMY 



the frog. This outgrowth is solid, is covered with splanch- 

 nopleuric mesoblast which soon becomes highly vascular, 

 and is an important embryonic organ known as the allantois. 

 Pushing its way between splanchnopleur and somatopleur, 

 the allantois grows up into the extra-embryonic ccelomic space 

 in the direction of the trager, and presently meets and 

 becomes attached to this organ. It then gives off numerous 

 branching upgrowths, the allantoic villi, which penetrate into 

 the tra'ger, and are surrounded by the trophoblastic tissue 

 of which that organ is composed. The allantoic villi are 

 highly vascular, being supplied with embryonic blood-vessels 

 by the allantoic artery. The lacunae of the trager are filled 

 with maternal blood, and thus an exchange of material is 

 readily effected between the maternal and embryonic blood, 

 whereby the embryo obtains nourishment and gets rid of 

 waste products. 



During this time the uterus undergoes remarkable changes, 

 represented in fig. 75, F and G. In F the young embryo 

 is seen lying in a pit in the lower or anti-mesometric side 

 of the uterus, and the uterine lumen is continued above the 

 pit. In consequence of the loss of the uterine epithelium 

 and adhesion of the trager to the uterine wall, the original 

 lumen above the embryo is obliterated, and a new lumen 

 is formed by outgrowths which pass through the thickened 

 uterine wall and meet below the embryo in the manner shown 

 in G. It can be seen that on the establishment of the new 

 lumen a part of the uterine tissue adjacent to the yolk-sac 

 is cut off from the rest, and forms a thick layer or envelope 

 below the embryo. This layer is lined by trophoblastic 

 tissue, and lacunar spaces containing maternal blood are 

 formed in it. At this time the yolk-sac is highly vascular, 

 being supplied by the omphaloidean artery, and thus 'the 

 embryo is receiving nourishment both by the agency of the 

 allantois and rite yolk-sac. The name placenta is given to 

 all trophoblastic structures in which there are lacunae con- 

 taining maternal blood, so at this stage (fig, 76) there are 

 two placentae present; the one connected with the allantois 

 is the allantoidean placenta, the one connected with the 

 yolk-sac is the omphaloidean placenta. 



As development proceeds the allantoidean placenta gains 

 in size and importance, the omphaloidean diminishes and 



