212 GENERATION. 



The animal would appear to begin at the back, as it contains the 

 spinal marrow, in which is to be included the head, as it contains the 

 brain, and it seems to build forwards, and the new parts are formed in 

 succession ; so there appears to be originally no outline of the whole, 

 and the parts to form in it ; therefore every part is formed on the out- 

 side of the animal : thus we see the heart, then the lungs, the intestines, 

 and over the whole the skin of the abdomen, which is not perfected till 

 the animal is ready to hatch, and sometimes not even then. 



As this only relates to the bird, it may be supposed to belong to it 

 only ; but there is reason to believe it is the same in other animals ; for 

 in some monsters, in the quadruped, we have no abdominal parietes, 

 only the bowels covered by a thin skin, which leads us to conjecture it 

 possible that they also are formed without any abdominal parietes. This 

 state of deficiency of the parietes of the abdomen has all its degrees, 

 some much more, others less. 



The chick is formed first on its back, and then turns on its left side ; 

 and till this period the heart is not seen, or if it exists it must lie before 

 the medulla, which will, from its transparency, render it obscure ; for 

 in this side view, we see, as it were, the profile, and from its lying in a 

 transparent fluid, it can be seen moving in it even before there is any 

 red blood 1 . 



Of the Blood's Motion in the Chick. 



The circulation of the blood in the foetus of the common viviparous 

 animals may be divided into two parts : the first is that which passes 

 immediately through both sides of the heart with the connexion between 

 the arteries of the right and left side of the heart. The second is that 

 which is connected with the membranes for the foetus's nourishment. 



In the oviparous animals the motion of the blood may be divided into 

 three ; first, as above, for instance, its motion immediately through the 

 heart, and the communication between the arteries of the right and left 

 side ; the second, as above, viz. the connexion with the membranes for 

 nourishment ; and the third (which is probably peculiar to them) is the 



1 [" The red globules appear not to be a natural part of the blood, but, as it were, 

 composed out of it, or composed in it, and not with it ; for they seem to be formed 

 later in life than the other two constituents ; for we see while the chick is in the egg 

 the heart beating, and it then contains a transparent fluid before any red globules 

 are formed, which fluid we may suppose to be the serum and lymph. Whatever 

 may be their utility in the machine, the red globules certainly are not of such 

 universal use as the coagulating lymph, since they are not to be found in all animals, 

 nor so early in those that have them."— Hunter, On the Blood and Inflammation, 

 4to. 1794, pp. 45, 40.] 



