324 Dr Allen Thomson on the Vascular System 



farther changes which take place in the development of the heart 

 of the foetus of mammiferous animals, as the structure of the 

 heart in the later stages of gestation has been made the subject 

 of several treatises, and as a German work has lately appeared 

 on this subject alone, in which all points connected with it re- 

 ceive a full discussion *. 



Explanation of the Plates. 



The individual parts of all the following figures are indicated by the letters 

 of the alphabet, as follows : — 



a, The transparent area of the cicatricula. b. The primitive trace of the 

 foetus, c, The serous layer of the germinal membrane, c', The mucous 

 do. c". The vascular do. rf, The primitive folds of the spinal canal. 

 d\ Their cephalic part, e, The head sheath or fold of the intestine. 

 /, The tail sheath or fold of the intestine, gg. The two lateral folds of 

 the intestine joining the head and tail folds, h, The heart, r, The au- 

 ricle, r'. The right auricle, s, The ventricle, a-*, The right ventricle. 

 t. The bulb of the aorta, or aorta. t\ The pulmonary artery, x. The 

 veins entering the auricle, i, The auriculo-ventricular valves. Ar, The 

 capillary vessels of the vascular area. /, The omphalo-mesenteric ar- 

 teries. to, The ductus arteriosus, w. The notch between the right and 

 left ventricles, o, The part of the yolk uncovered by the germinal mem. 

 brane. «, The eustachian valve, z, The septum between the ventricles. 



PLATE II. 



1. BlllDS. 



Y'lS. 1. The impregnated egg of the Common Fowl, with the cicatricula on 

 the surface of the yolk. 



2. The cicatricula magnified about five diameters. 



3. Vertical section of the cicatricula. 



4. The cicatricula with the primitive trace, shewing the position of 



this trace in relation to the yolk as represented in Fig. 1. 

 6. The primitive trace in the transparent area, and the commencing 

 primitive folds, after 14 hours' incubation. 



6. The same after 18 hours. 



7. (From Baei-) Vertical sections of the cicatricula in the longitudinal 



and transverse diameter of the foetus. 



8. The primitive folds of the spinal canal closed behind at the 24th 



hour. 



• Kilian. Ueber den Kreislauf des Blutes im Kinde welches noch nicht 

 gcathraet hat. Karlsruhe, 1826. 



