VERTEBRATES FROM A ORUSTAOEAN-LIKE ANCESTOR. 411 



The observations of Hunter, then, point to the conclusion 

 that in health wherever the blood tends to stagnate the con- 

 ditions are most favorable for passive destruction, and therefore 

 for the accumulation of pigment. As he shows, all active cells 

 in immediate relation to the stagnating blood play a part in 

 converting the haemoglobin of the original blood-corpuscles 

 into blood-pigment. Around extravasated blood it is the cells 

 of leucocytes and of connective-tissue nature which carry on 

 this process ; in the spleen and bone marrow the still more 

 active splenic and marrow cells. 



The same conditions as are present chiefly in the spleen and 

 red bone marrow in the case of the higher Vertebrates occur 

 freely in different parts of the body in the Ammocoetes owing 

 to the peculiarities of its circulation, for the characteristic of 

 the vascular system of the Ammocoetes, especially before trans- 

 formation, is the abundance of large blood spaces in different 

 parts of the body. These blood spaces have been somewhat 

 strangely described as lymph spaces containing red blood- 

 corpuscles. In them, as in the spleen pulp, the blood circu- 

 lates slowly and with difficulty. In addition, important and 

 extensive changes in the arrangement of various parts of the 

 body take place during metamorphosis. In fact, all the con- 

 ditions necessary for passive blood destruction are present in 

 the most pronounced manner. 



The examination of the AmmoccEtes by dissection and sections 

 shows that masses of pigment are found in very definite arrange- 

 ments in many different regions of the body. Apart from such 

 places as the eye and the skin, we find pigment most markedly 

 in connection with the branchiae and with the pronephros. In 

 both these cases the pigment is found in close connection with 

 the vascular arrangements. Thus the tubules of the pronephros 

 of the Ammocoetes are enclosed in a large lacunar blood space, 

 the walls and septa of which are strongly pigmented. In the 

 branchial regions masses of pigment are found forming a 

 coarse network, the meshes of which are full of blood-cor- 

 puscles, so that the pigment here, just as in the pronephros, is 

 situated in the walls of the vascular system. The heart itself 



