MEMOIR OP BARON HALLER. 47 
vessel, is alone sufficient. Instead of water and 
other perfectly fluid elements, the enlarged vessels 
transmit particles which are viscid and albuminous, 
which mutually attract each other, and the nearer 
they approach the stronger they attract. 
The manner in which the. parts from being in- 
visible become visible, is truly simple ; it is effected 
by enlargement, and still more by opacity. The 
lungs become visible only on the sixth day. When 
first perceived, they are sixteen-hmidredth parts of 
an inch long; they might have been visible when 
only four-hundredth parts long ; but they are not 
when eight-hundredth parts, solely because they 
were diaphenous, and of the same colour as the 
other parts. The liver is still larger on its first ap- 
pearance ; and if it does not appear earlier, it is not 
owing to its small size, hut to its want of opacity. 
It is the same with other parts ; so that we should 
he cautious in supposing that any portion of an 
animal is newly created, or that it had no previous 
existence : it may have been too small for observa- 
tion, or may have been transparent. 
The movement, and apparent repose of the parts 
of the body, depend also on the increase and opacity 
of the parts. The heart does not appear to have 
any movement previous to the lapse of forty-eight 
hours. Why, it may he inquired, does motion then 
appear ? and is it not true that the heart has pre- 
viously propelled the fluids with vigour, since the 
growth of the chick has been so rapid ? If the heart 
has appeared in repose, it has been because it was 
