DEVELOPMENT OF THE CHICK. 207 



When heat is applied to an impregnated egg, the living parts are put 

 into motion, and an expansion of what is called the cicatricula takes 

 place. This very probably begins at the chick as a centre ; but it 

 would appear that the whole did not derive its expansion immediately 

 from the chick, for this part would appear to have powers within itself, 

 and the further from the chick these powers are at an early period, the 

 strongest is this expansion of parts ; for we find changes taking place 

 in this circle near to the circumference 1 , sooner than near to the chick, 

 which afterwards become distinct vessels, and communicate with the 

 mesenteric artery of the chick. The chick begins to take form to 

 itself in the midst of this expansion, and as it increases, its influence 

 is extended into the surrounding parts. 



In the beginning of the formation of the chick, there is great di- 

 stinctness of parts, for they gradually take place one after another. 



Of the Membranes of the Chick. 



The chick at first, or in its hour, is totally void of mem- 

 brane, only having over it the external membrane of the yolk 2 , which, 

 when removed (which is easily done), the animal is perfectly bare 3 . 



The first formation or expansion of its membranes are in pretty 

 quick succession, and then go on together, some being sooner completed 

 than others. The first expansion of parts would appear to be the 

 formation of membranes, or changes in membranes naturally belonging 

 to the egg. 



The first membrane that is formed is the membrana vitelli 4 , which 

 forms immediately under the proper membrane of the yolk ; so it 

 would appear that at this time the yolk had two membranes (but how 

 far originally so I do not know), the external, a fine transparent one, 

 and the other, more spongy, and having the power of becoming 

 vascular. 



As the parts of the chick begin to form, such as the head and spine, 

 with the medulla spinalis, &c, a proper membrane also begins to form, 

 to cover it. This membrane 5 begins first at the head, and seems to 



1 [The formation of the halones~ and blood-lakes in the area vasculosa is here 

 alluded to.] 



2 [The membrana vitelli, or cuticula vitelli.'] 



3 [In the embryo of the common fowl the amniotic investment begins to be 

 formed at the eighteenth or twentieth horn*, but is not completed until the fourth day.] 



4 [The 'blastoderm' or 'germinal membrane,' and not the 'membrana vitelli' of 

 modern embryologists.] 



5 [The ' serous layer ' of the germinal membrane : it is also called the ' animal 

 layer ' by some embryologists ; but that the serous layer is the covering, and not the 



