250 Professor Clark on a Case 



the whole united portions of the two bodies, shews that if such 

 an union did take place it must have been in corresponding parts. 

 These parts I conclude to have been in front of the primitive 

 streaks of the embryos. Let us now suppose that the evolution of 

 each embryo advances, that the dorsal plates and spinal cavities 

 and vesicles of the brain are formed in each : until the space 

 between the anterior extremity of each is occupied by the anterior 

 vesicle of the brain of each. These, according to M. Serres' law, 

 will now coalesce, and our dissection shews that these are the 

 only parts which did coalesce, for the cerebella and corpora qua- 

 drigemina were separate and distinct. The ventral plates of the 

 opposite embryos necessarily meet in front: and there is an effort 

 to bend the head downwards to form the cavity for the heart. 

 This bending takes place in the direction of the ventral plates, 

 the effort being resisted in every other. Thus there are cavities 

 for the hearts properly formed : and they receive their blood from 

 opposite sides of the united vascular areas. Let us now consider 

 how the aortas are to be formed. Five pairs of branchial arches 

 project from each heart on either side of the neck. They arch 

 round to the basis of the brain to reach the situation of the spinal 

 cord, and there coalesce to form the roots of the descending 

 aorta. Thus the descending aorta will, in this case, be formed by 

 branchial arteries which come from different hearts: they are the 

 root of the third pairs, which arches from either heart in a di- 

 rection from right to left, until it gains the spine. Thus all the 

 arteries which arise below the original third branchial artery, 

 come from the descending aorta, whilst the arteries of the head, 

 come from the arch which connects the descending aorta to the 

 heart. And that distribution is exactly the one before us: the 

 aorta has reached the spine to which it does not seem to belong, 



