in the Fatus of Vcrtchrated Animals. 323 



tached to one another in a greater space towards their bases, 

 and the notch between them is thus diminished. The apex of 

 the heart still appears double at the tenth week, (Fig. 16) ; but 

 at the end of the third month, the ventricles are very little se- 

 parated from one another, though the place whei-e the notch 

 previously existed is still strongly marked (Fig. 17.) 



We have already remarked, that there is at first only one ves- 

 sel which arises from the ventricle of the heart ; this is the bulb 

 of the aorta, which joins with the ventricles immediately above 

 the septum, and seems to communicate freely with both till the 

 seventh week. About the seventh or eighih week, the bulb of 

 the aorta becomes gradually divided into two vessels, for the for- 

 mation of the ascending part of the aorta proper, and the root of 

 the pulmonary arteries, (if, Figs. 16, 17). The division takes 

 place first towards the ventricle, so that at first, when the lower 

 part is divided, the higher still remains single. The division 

 proceeding upwards does not separate the whole of the bulb of 

 the aorta into two vessels; a communication is left at the upper 

 part, (m, Fig. 16), by means of which the ductus arteriosus is af- 

 terwards formed. 



The left auricle is at first much smaller than the right ; and 

 at the time of its first separation, as in that of the chick, we can 

 discover no connexion between it and the pulmonary veins. 

 In the seventh week, the anterior parts of the auricles are 

 prolonged forwards, so as to place themselves at the base of 

 the ventricles, and to cover anteriorly the bulb of the aorta, 

 or origin of the pulmonary and aoriic trunks. At this time 

 the vena cava inferior appears to enter the left auricle (the 

 superior vena cava leading into the right, as in the adult), 

 as the back part of the septum of the auricle is at this time 

 formed by the eustachian valve (Fig. 20). At the ninth 

 week the proper septum of the auricles begins to appear a little 

 to the left of the vena cava inferior, and the foramen ovale is 

 seen quite open between this, the lower, and the upper part of 

 the septum auricularum, which descends from above. The eus- 

 tachian valve is still very large ; at the end of the third month, 

 it decreases in size, and the valve of the foramen ovale begins to 

 be formed. 



It appears to mc to be unnecessary to continue to describe the 



