74) Dr Prout’s Experiments on the Changes which take place 
3. That in the earlier stages of incubation, an interchange of 
principles takes place between the yolk and a portion of the 
albumen ; that this interchange is confined on the part of the 
yolk to a little of its oily matter, which is found mixed with the 
above mentioned albumen ; that this portion of albumen under- 
goes some remarkable changes, and is converted into a sub- 
stance analogous in its appearance, as well as in some of its pro- 
perties, to the curd of milk ; and, lastly, that a portion of the 
watery and saline portion of the albumen is found mixed with 
the yolk, which becomes thus apparently increased in size. 
4. That as incubation proceeds, the saline and watery parts 
again quit the yolk, which is thus reduced to its original bulk ; 
* that in the last week of the process it undergoes still further 
diminution in weight, and loses the greater portion of its phos- 
phorus, which is found in the animal converted into phosphoric 
acid, and in union with time^ constituting its bony skeleton ; 
and, lastly, that this lime does not originally exist in the recent 
egg, but is derived from some unknown source during the pro- 
cess of incubation. 
These, and other interesting circumstances, arising out of 
the present inquiry, suggest some important hints respecting 
certain operations of the animal economy. They also serve to 
direct the attention of the microscopic inquirer to the investi- 
gation of points, which it is probably within his power to eluci- 
date, but of which, at present, we are ignorant. This part of 
the subject, however, cannot be in abler hands than it is at pre- 
sent. To the distinguished physiologists, therefore, who have 
been recently engaged in the investigation, I willingly leave it, 
and shall conclude with a few remarks only on the uses of the 
yolk, and the apparent generation of earthy matter. 
Sir Everard Home and Mr Hatchett have concluded, from 
their experiments, that the yolk is analogous to the milk of vi- 
viparous animals, but more concentrated, and that its chief use 
is to afford a pabulum to the young animal during incubation *. 
This opinion, which is indeed as old as Aristotle f, is corrobo- 
• Phil. Trans. Vol. cvi., p. 301., et sey. 
f *H jttsv ovv T °v ve&rrou hnv hi rov tevxov, Tz r^o(pv) dto& tov 
optyaXov he rov mxqov. Aristotelis de Animal, Hist. vi. 3. (Ed. Schneider-) 
