426 IMPLANTATION OF THE OVUM IN 



the trophoblast is complete and structurally similar at all points. There must now 

 be distinguished in the blastocyst what may be called the placental or embryonal pole, 

 the point where the inner cell-mass is attached; and on the opposite side what will 

 be referred to as the fixation-pole (Gegenpol, Spee). This is not necessarily diamet- 

 rically opposite to the embryonal pole, but is approximately so. The first change 

 noted at the fixation-pole is a thickening of the cell-layer, the loss of cell-boundaries, 

 and an increase and enlargement of the nuclei until a syncytium, or giant cell-mass, 

 the fixation-mass, has been produced which projects from the trophoblast layer at 

 this point. Compare Figures 10, 14, 16, and 29, where the fixation-mass is seen in 

 close proximity to the uterine epithelium, but has not yet penetrated it. It will 

 be noted that the cells of the trophoblast on either side of the fixation-mass are not 

 altered, but continue to divide rapidly and extend as flattened cells, causing an increase 

 in the size of the blastocyst. Where the fixation-mass comes into contact with the 

 uterine epithelium there is a breaking down of the epithelium, a dissolving or absorp- 

 tion process. Very early stages (PL XXX, Fig. 16; PL XXXI, Fig. 29) show a 

 depression caused by the destruction of the free end of the epithelial cells. One is 

 reminded frequently of the formation of Howship's lacunae in developing bone by 

 the osteoclasts. This process continues and the fixation-mass bores or eats its way, 

 as it were, through the epithelium until it comes into contact with the subepithelial 

 connective tissue (PL XXX, Figs. 20, 21; PL XXXI, Figs. 27, 28). In a slightly 

 older stage (PL XXX, Fig. 19) the entire fixation-mass has passed through the 

 epithelium. The fixation-mass now becomes convex on the surface next to the con- 

 nective tissue, while the edges become much thinned and begin to extend between 

 the epithelium and connective tissue (PL XXX, Fig. 23; PL XXXI, Figs. 30, 31). 

 3. Completion of the Process of Fixation. — A. Uterus. — Both external and internal 

 changes are to be noted in the uterus. The uterine tube is increasing in length and 

 diameter, and now swellings begin to show externally which indicate the points of 

 attachment of the blastocysts. These swellings are most marked on the ventral 

 side of the uterine horns, the mesometrial portion not being affected until later. 

 Internally there is a further increase in the capillaries, many of them are dilated and 

 in closer relation with the fixation-mass (PL XXX, Fig. 19; PL XXXI, Fig. 34), 

 while thickenings are observed in many of the endothelial cells. The connective- 

 tissue cells are increasing in number and in volume; their edges, slightly thickened 

 and closely packed together, present an epithelial-like appearance. The fixation- 

 chamber is increasing in size to accommodate the enlarging blastocyst. This is accom- 

 plished at the expense of the intermediate portion, whose lower margins are gradually 

 incorporated into the upper part of the fixation-chamber. The dilated ends of the 



