DIGESTIVE ACTIVITY OF MESENCHYME 445 



Changes in the allantois during the last three days of incubation 



Regressive changes are found in the vascular net of the allan- 

 tois about the eighteenth day of incubation, and on the nine- 

 teenth day parts of the capillary network extending over the 

 serosa are found collapsed or disappeared altogether over large 

 areas. The ectoderm of the serosa, which at definite stages 

 occupied a deeper position in the membrane, now again be- 

 comes superficial. A curious vacuolization of nmnerous ecto- 

 dermal, cells is observed and the epithelial layer of the serosa 

 becomes thickened. 



Conspicuous changes develop in the walls of the vessels, espe- 

 cially of the arteries. In the earlier stages the walls are very 

 thin with hardly perceptible muscular coats, while in the latter 

 stages of incubation they become thick with heavy layers of 

 muscular tissue. The partial destruction of the superficial capil- 

 lary net might well explain the hypertrophy of the muscular coat 

 of the arteries and a consequent increase of the blood-pressure in 

 them. Another immediate consequence of the destruction of the 

 capillary net is the development of an edema through the allantois, 

 which in the last days of incubation is many times thicker than 

 in earlier stages. Its mesodermal tissue contains few cells and 

 the vascular channels are widely separated. The edema, a 

 natural consequence of the destruction of the capillary net, 

 itself becomes a factor in the further destruction of this net. 

 These changes in the allantois coincide with the arrest of growth 

 in the graft and are of a character to suggest an inhibiting cause. 



Striking changes are frequently observed in the vascular net 

 of the graft itself. Greatly distended capillaries are seen to be 

 engorged with stagnating blood (figs. 3 and 4). The red cor- 

 puscles lose their characteristic structure, their nuclei disinte-. 

 grate and are no longer discernible. Sometimes they fragment; 

 in other cases they flow together and form large masses of a sub- 

 stance showing the staining reaction of hemoglobin. The endo- 

 thelial walls of the vessels break down, individual endothelial 

 cells begin to show phagocytic activity and ingest the huge masses 

 of protein substance so formed. Red blood corpuscles reduced to 



