440 



AN INTRODUCTION TO ZOOLOGY 



enlarge greatly in early pregnancy, and the ridge capillaries dilate 

 considerably. The uterine epithelium in touch with the trophoderm 

 disintegrates and is absorbed, and the syncytium actively grows 

 down into the underlying mucosa in the form of a series of thin plates 

 arranged in the form of an irregular honeycomb. These grow in, 

 absorbing the tissue of the mucosa as they do so, and come to sur- 

 round a number of the maternal capillaries. Finally, even the 



Pi. 



f.yk. 



FIG. 155. 'Diagrammatic transverse section of a pregnant uterus, 

 illustrating the later phase of placentation. From Bourne. 



all., allantois ; am., amnion ; bv., uterine blood-vessels; em., embryo; gl., uterine glands 

 lu' 2 , secondary lumen of uterus ; ms.m., mesometrium ; mus., muscular wall of uterus ; PL, 

 allantoidean placenta ; p.yk., proximal wall of yolk sac ; umb., stalk of yolk sac ; Ut., uterus. 



walls of the capillaries break down, and so the parental blood comes 

 to circulate in a series of sinus-like spaces in the trophoderm. 



While this has been going on the allantois has grown out and 

 fused with the chorion, bringing with it its mesoderm in the form of 

 a richly vascularised mesenchyme. This mesenchyme grows down- 

 wards between the trophoblastic lamellae, and so forms a series of long 

 papillae, known as villi. Thus we have a series of tongues of mesen- 

 chyme containing embryonal blood-vessels and surrounded by the 

 trophoblastic tissue, in whose sinuses circulates the maternal blood. 



