122 CARNEGIE INSTITUTION OF WASHINGTON. 



between "passive migration," in which there is a change in position due 

 to some flexion or traction on the vein wall itself, and "spontaneous 

 migration," in which there is a change in position of the blood-stream 

 only, whereby by circumfluent anastomosis the blood-stream develops 

 a new channel in the adjacent loops of the plexus, whereupon the pre- 

 vioush^ used loop correspondingly dwindles. 



In the adjustment of the drainage of the head there also occur 

 instances of the formation of new replacement channels with consequent 

 obliteration of old ones — which process, however, can not be classed as 

 spontaneous migration. It is a change in position, but the change is an 

 abrupt one. The new channel turns off at once into an entirely new 

 direction. Moreover, the new channel lacks the morphological char- 

 acteristics of the old one. As in spontaneous migration, this change 

 is accomplished by circumfluent anastomosis. An example of this 

 type of adjustment is seen in the otic region. Owing to the growth of 

 the cochlea and the structures of the middle ear the course of the primary 

 head- vein, ventro-lateral to the otic capsule, becomes an unfavorable 

 one and this portion of the primary head- vein becomes obliterated. 

 Adjustment is made for this in two ways. First, a channel is estab- 

 lished dorsal to the otic capsule through which the middle dural plexus 

 is drained caudally into the posterior dural plexus. Secondly, the 

 anterior dural plexus, which originally drained into the primary head- 

 vein, fuses with the middle dural plexus and drains caudally through 

 this and thi'ough the new channel dorsal to the otic capsule. This 

 makes a complete trunk for the drainage of the head, which is through- 

 out its course dorsal to the primary head-vein as far caudalward as the 

 jugular foramen, where it is continuous with the jugular vein. This 

 trunk becomes the transverse sinus and can not be regarded as a 

 migrated primary head- vein, though to a large extent it replaces the 

 latter. 



As a final variety of adjustment in the veins of the head there is the 

 formation of entirely new veins, which become necessary for the drain- 

 age of structures that are late in making their appearance. It is found 

 that in their development the veins of the two sides of the head are 

 asymmetrical and that the sagittal plexus in embryos from 20 mm. 

 upward, in about 90 per cent of the embryos examined, drains pre- 

 dominantly toward the right side. 



Professor Clark has published studies on the growth of blood-vessels 

 in the tail of the tadpole. By direct observations he has been able to 

 add confirmatory evidence to the Aeby-Thoma conception of the adap- 

 tive capillary formation of blood-vessels. The arterioles and venules 

 develop from an indifferent capillary plexus, any capillary of which may 

 become incorporated in the advancing arteriole or venule, depending 

 upon whether it is favorably or unfavorably placed as regards the more 

 peripherally situated capillary mesh. In the embryonic chick he sue- 



