394 H. H. Newman and J. T. Patterson. 



common amnion into a single pocket and leaves for a short dis- 

 tance a single connecting canal. Later each member of these pairs 

 loses its connection with its partner and acquires its own canal. 

 This secondary separation of the pairs produces a forking of each 

 of the original two connecting canals, a condition that persists 

 for a long time. 



After the embryos have left the common amnion the latter 

 probably becomes functionless and ceases to grow. Fortunately 

 however it persists with all of its connections through a consider- 

 able developmental period, furnishing evidences of polyembryony 

 and of embryonic pairing. In fig. 44 it is shown still typical in 

 form with its connecting canals entire but with their lumens in- 

 terrupted with plugs of tissue. The regions between the plugs 

 have become distended through local secretion of ammo tic fluid, 

 so that the canals as a whole present a decidedly moniliform appear- 

 ance. In fig. 45 a somewhat more advanced stage of degenera- 

 tion in these structures is seen. The common amnion can no 

 longer be recognized but the canals are still clearly defined. 

 Each of these shows a number of pronounced bead-like swellings, 

 one of which may represent the remains of the common amnion. 

 These canals may persist until stages as advanced as that shown 

 in fig. 33, but are seldom to be detected in later stages. 



The posterior amniotic processes, which in early stages were 

 seen to be closely associated with the development of the allan- 

 tois, do not persist in so marked a form in our species as in the 

 Mulita. Only in rare cases does one see any traces of these 

 structures at a period later than the five to seven somite condition 

 (fig. 15). In vesicle 17, however, one of the embryonic amnia 

 is connected by means of an amniotic canal with a sac as large or 

 larger than the common amnion but lying at the opposite pole 

 of the vesicle. This condition is no doubt exceptional and may be 

 accounted for on the supposition that the posterior amniotic 

 process of one of the embryos, on account of its unusual length, 

 protruded far down into the Trager region, came into contact 

 with and united with it, and subsequently swelled into an amniotic 

 sac at the point where its terminal bulb fused with the Trager 

 wall. 



