BEHAVIOR OF ANURA TOWARD COLLOIDAL DYES 45 



A blood-vascular circuit is in active operation through heart, 

 arteries, capillaries and veins. All of the blood from the posterior 

 regions is returned to the heart by the postcardinal veins. The 

 proximal or hepatic portion of the future postcava lies in direct 

 contact at the hepatico-subcardinal junction with the right sub- 

 cardinal vein (McClure, '06, p. 168). As a communication has not 

 been established between these two veins at this juncture, a 

 postcaval circulation has not yet been formed. The external gills 

 are highly developed and their capillary circulation active. Small 

 loops of blood-vessels extend for a short distance into the caudal 

 fins. No lymphatics, however, are found in the fins. 



The thyroid is detached from the floor of the pharynx but is not 

 yet completely separated into a right and left gland. A number 

 of pronephric tubules are present but only two nephrostomes for 

 each kidney have been formed. The right mesonephros is repre- 

 sented by three small solid anlages but no hollow renal vesicles 

 have been formed. The anlages of the left mesonephros are much 

 smaller and less distinct than those on the right side. 



White blood-corpuscles are met with in various stages of dif- 

 ferentiation in the general blood-vascular circulation, pronephros 

 and mesenchyme. Many of them are in an active state of division 

 and some of them are approaching the typical mononuclear form. 

 The general impression one obtains is that none of these cells have 

 completed tjieir differentiation and that none of them could now 

 be regarded as typical leucocytes. No indication of any cells 

 arising from lymphatic endothelium was observed at this stage. 



The distinctive features observed on the fifth day, are the dis- 

 connected character of the anlages of the lymphatics in the head 

 region and the undifferentiated character of the white blood-cells. 

 It is also to be noted that one of the main blood-vascular channels 

 has not been established, as the postcava is not yet formed. 



On the sixth day after fertilization in Bufo marked changes in 

 the organs of circulation are observed. The original and early 

 independent anlages of the ventral cephalic lymph sinus are no 

 longer found. These anlages have now become conjlmnL io forni-a 

 large single-chambered sinus which occupies the position of t he 

 submaxillary lymph sinus in the adult. This lymph sinus of the 



