in the Feet us of Vcrtebratcd Animals. SI 3 



3. Development of the Heart of Birds. 



The appearance of the heart of the Chick^ at the earliest pe- 

 riod at which it is visible, has not been described exactly in the 

 same way by the different authors who have observed it. Baer * 

 describes the rudimentary heart as having the appearance of a 

 dilated vessel situated between the mucous and serous layers on 

 the lower side of the oesophagus; while Pander -f-, Prevost, and 

 Dumas J, compare it to a sac or pouch closed at the antei'ior 

 extremity, situated between the mucous and vascular layers of 

 the germinal membrane ; and describe it as somewhat similar in 

 its form to the CESophagus itself (Fig. 12, A and B.) From 

 the drawings of it which Pander, Prevost, and Dumas have 

 given, I am inclined to think that the view which they have 

 taken of the subject is the more correct one, so far as regards 

 the form and position of the heart ; but I do not think it pro- 

 bable that the anterior extremity of this organ is shut or closed. 



The heart of the chick retains the appearance of a sac for a 

 short time only ; its anterior part is soon produced forwards, 

 while the middle part of the cavity of the organ is narrowed so 

 as to reduce it to a tubular form (35th hour). The posterior 

 part at the same time sends out a prolongation on each side of 

 the foetus into the transparent area ; and these prolongations, 

 formed at first only of globules loosely connected together, in- 

 dicate the place where the veins afterwards enter the heart, 

 (Fig. 12, A). 



At first the tube of the heart can only be seen on looking at 

 the abdominal side of the embryo, but towards the 30th or 33d 

 hour, it is dilated in its middle, and is bent out towards the 

 right side, so that part of it can be seen when the foetus is viewed 

 from above, as is the case when it lies on the surface of the yolk 

 in the egg. The tube of the heart remains as yet simple and 

 undivided ; and so long as motion has not commenced, nothing 

 remarkable distinguishes one part of it from another. We may 

 state, however, that the posterior part, or that towards the tail, 



" Entwickelungsgeschichte der Thiere. Konigsberg. 1828, and in Bur- 

 dach's work, vol. ii. -j- Loc. citat. 



X Sur la Formation du Coeur dans le Poulet. Annal. des Scien. Natur. 

 torn. iii. ; and, Sur le Developpement du Poulet, ibid. torn. xii. 



JULY SEPTEMBER 1830. X 



