THE INNER TISSUES OF ANIMALS. 333 



of a simple animal, made to ooze now in this direction and 

 now in that by osmotic distension and by the changes of 

 pressure which the animal's movements cause, comes to have 

 certain lines along which it is thrust backwards and forwards 

 more than along other lines ; and must by repeated passings 

 make these more and more permeable, until they become 

 lacunae ? Such actions will inevitably go on; and such actions 

 appear competent to produce some, at least, of the observed 

 effects. The leading facts which indicate that this is a part 

 cause of vascular development, are these. 



Growths normally recurring in certain places at certain 

 intervals, are accompanied by local formations of blood-vessels. 

 The periodic maturation of ova among the Mammalia, supplies 

 an instance. Through the stroma of an ovarium are dis- 

 tributed innumerable minute vesicles, which, in their early 

 stages, are microscopic. Of these, severally contained in their 

 minute ovi-sacs, any one may develop : the determining 

 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 currents towards the point of growth determines the 

 formation of the blood-vessels. It may be that having 

 once commenced, this local vascular structure completes 

 itself in a typical manner ; but it seems clear that this 

 greater development of blood-vessels around the growing 

 ovum is initiated by the draught towards it. Ab- 



normal growths show still better this relation of cause and ef- 

 fect. The false membranes sometimes found in the bronchial 

 tubes in croup, 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 



