748 



ANATOMY OF VERTEBRATES. 



where the decidua ends. The utricular canals become dilated 

 and tortuous, and are still lined (or formed) by epithelial cells : 

 but formifaction is active in the production of diverse defined cor- 

 puscles from the ( granule' up to the colossal * fibre-cells,' fig. 416. 

 At the fourth or fifth month the decidua becomes condensed to 



a thinner layer, and detached 

 from the muscular wall of the 

 uterus by a new, soft deposit, 

 which takes on the utricular 

 character of the original lining 

 substance, and remains after 

 parturition. 



The primary changes of the 

 impregnated human ovum 

 have not been observed. It 

 cannot be doubted that the 

 germ-mass is due to the cleav- 

 age process. Whether the 

 outer coat continues to be 

 the hyalinion when the ovum 

 passes into its deciduous nest ; 

 whether the hyalinion then 

 gives place to an expanded 

 ( animal ' layer of the blasto- 

 derm ; or whether this be superseded by the vascular layer of 

 the allantois — are conjectural possibilities suggested by observed 

 facts in lower Mammals, and awaiting proof. This is certain, 

 that when the amnion is completed, the intestine formed, and 

 the vacancy of the ventral walls contracted to an umbilicus, the 

 remnant of the vitellicle is reduced to a crumpled yellowish sub- 

 circular corpuscle, 1^ line in diameter, adherent to the outside of 

 the amnion, and connected with the intestine by a long filamentary 

 omphalo-mesenteric pedicle, accompanied by a vitelline vein and 

 arteries. The vascular layer of the allantois has formed, or or- 

 ganised, the chorion : its unvascular layer is disposed like a serous 

 membrane between the amnion and chorion, and maintains a con- 

 nection for a time with a filamentary urachus, expanding within 

 the pelvis into a urinary bladder. The growing ovum pushes the 

 free wall of its decidual chamber into the uterine cavity (traversed 

 by the bristles in fig. 581), and, filling it, reduces it to a narrow 

 ' hydroperionic space.' The layer of decidua so pushed in seems 

 to be reflected upon the ovum, and is termed i decidua reflexa ' or 

 \ decidua ovuli ': the thicker layer lining the womb is the ( decidua 



Decidual lining substance of human uterus, shed in 

 abortion, ccxlvi". 



