THE INNER TISSUES OF ANIMALS. 343 



cause being probably some slight excess of nutrition. When 

 the development is becoming rapid, the capillaries of the 

 neighbouring stroma increase and form a plexus on the walls 

 of the ovi-sac. Now since there is no typical distribution of 

 the developing ova; and since the increase of an ovum to 

 a certain size precedes the increase of vascularity round it; 

 we can scarcely help concluding that the setting up of cur- 

 rents towards the point of growth determines the forma- 

 tion of the blood-vessels. It may be that having once com- 

 menced, this local vascular structure completes itself in a 

 typical manner; but it seems clear that this greater develop- 

 ment of blood-vessels around the growing ovum is initiated 

 by the draught towards it. Abnormal growths show 



still better this relation of cause and effect. The false mem- 

 branes sometimes found in the bronchial tubes in inflam- 

 matory diseases, may perhaps fairly be held abnormal in but 

 a partial sense: it may be said that their vascular systems 

 are formed after the type of the membranes to which they are 

 akin. But this can scarcely be said of the morbid growths 

 classed as malignant. The blood-vessels in an encephaloid 

 cancer, are led to enlarge and ramify, often to an immense 

 extent, by the unfolding of the morbid mass to which they 

 carry blood. Alien as is the structure as a whole to the type 

 of the organism; and alien in great measure as is its tissue 

 to the tissue on which it is seated; it nevertheless happens 

 that the growth of the alien tissue and accompanying ab- 

 straction of materials from the blood-vessels, determine a 

 corresponding growth of these blood-vessels. Unless, then, 

 we say that there is a providentially-created type of vascular 

 structure for each kind of morbid growth (and even this 

 would not much help us, since the vascular structure has 

 no constancy within the limits of each kind), we are com- 

 pelled to admit that in some way or other the currents of 

 blood are here directly instrumental in forming their own 

 channels. One more piece of evidence, before cited 



as exemplifying adaptation (§ 67), may be called to mind. 



