THE DEVELOPMENT OP THE MOLE. ' 155 



of the amnion, however, is formed only of epiblast and hypo- 

 blast, and has been called by van Beneden and Ch. Julin, who 

 first described this structure, the " pro-amnion." 



The Allantois commences in Stage f as a short wide diverti- 

 culum projecting upwards and backwards into the primitive 

 streak. This diverticulum enlarges during Stages g to j ; it 

 is lined with hypoblast cells (figs. 35 and 50). 



The Arterial System. — The dorsal aortse commence in Stage f, 

 and remain double until after Stage j ; they are connected with 

 the heart by a single pair of aortic arches during Stages h 

 and J, and give off vitelline arteries at their posterior end. 

 Internal carotid arteries and vertebral arteries are formed, and 

 it is from the latter of these vessels the mesoblast cells are 

 derived which migrate into the walls of the neural canal. 



The Venous System is very slightly developed. Vessels are 

 to be seen in the splanchnopleure over the yolk-sac at an early 

 date, but vitelline veins connected with the heart are not seen 

 until Stage h. Two short anterior cardinal veins are present 

 in Stage j, and traces of two posterior cardinals, but nothing 

 more. 



The Heart, which is formed of two tubes widely asunder in 

 Stage E, is composed of a single tube for a short distance in 

 Stage H, and is somewhat longer, but still straight and without 

 sign of division into chambers at the close of Stage j. The 

 thickened splanchnic mesoblast which gives rise to the heart, 

 splits into two layers at an early ag§. The outer of these 

 layers forms the outer wall of the heart, the inner the flattened 

 epithelium of the cavity of the heart. 



When the heart enlarges, as it does rapidly, a wide space 

 exists between these two layers, but they are connected together 

 by exceedingly fine processes of their cells which stretch across 

 the space. 



The Blood-Corpuscles appear to be formed from stellate 

 mesoblast cells directly. 



In conclusion, I may mention that I propose eventually to 

 follow the further development of the organs of the Mole, one 

 by one, and in doing so, to pay more attention to the researches 



VOL. XXVIl, PART "4. NEW SER. M 



