137 



this process comraeuces iu the centre and gradually extends out- 

 wards. Secondly the cells on the outer side become continuously 

 more and more flattened. The simultaneous flatteniug of the tro- 

 phoblast has already been mentioned. The decrease in the thick- 

 ness is however also due to a decrease in the number of layers 

 of cells present here, which in turn must be attributed to the 

 contiuued stretching of the whole layer consequent upon the growth 

 of the embryo inside. At about the eleventh day the capsularis 

 becomes separated from the anti-mesometric wall of the uterus 

 by the formation of a new lumen; it is here covered by a layer 

 of flattened cells similar in every respect to the remaining cells. 

 On the other side the new lumen is bounded by an ordiuary 

 columnar epithelium. The blood vessels of the capsularis and the 

 lacunae of the omphaloidean trophoblast still retain the blood 

 which was circulating in them before the degeneration set in; 

 and when these layers finally break a certain amount, very often 

 a considerable amount, of blood is set free unto the lumen of 

 the uterus by the rupture of the large venous sinuses which have 

 already been mentioned as radiating outwards aud upwards from 

 the border line between omphaloidean and allantoidean trophoblast. 

 Indeed it is no infrequent occurrence to find between the embryos 

 masses of blood, mixed up with degenerating megalokaryocytes, 

 which has been shed into the uterus in this way. 



Together with the capsularis and adjacent trophoblast there dis- 

 appears also the distal wall of the yolk-sac, and thus the lumen 

 of the yolk-sac is brought into open communication with the 

 lumen of the uterus. The proximal wall of the yolk-sac is covered 

 in its upper portion by a columnar epithelium (Fig. 20) thrown 

 into numerous folds penetrated by the capillaries of the area 

 vasculosa. The cells of the epithelium are a little irregular in 

 their height and have rounded outer ends; they contain numerous 

 large globules of fat and in addition small granules which resemble 

 very closely in their behaviour to fixing and staining reagents 

 the granules of the megalokaryocytes already described. Possibly 

 they are granules of zymogen. 



10 



