52 



THE BIOLOGY OF TWINS 



Tr-tl 



the stages shown in Figs. 15 and 17, the diagrammatic 

 Fig. 16 is shown. Here the embryos, which are in 

 an early primitive-streak stage, have migrated down 

 the meridians of the vesicle and have left behind 

 them thin-walled amniotic connecting canals that still 



anchor the head 

 ends of the em- 

 bryos to the small 

 common amnion 

 {c am) situated at 

 the distal or vege- 

 tative pole of the 

 egg. The posterior 

 end of each em- 

 bryo is now grow- 

 ing rapidly toward 

 the rim of the 

 Trager ring, a 

 junction with 

 which is soon to 

 be established. 



The sectional 

 view shown in 

 Fig. 14 indicates 

 that the heads of 

 embryos II and IV are growing apart, leaving between 

 them a thin sheet of ectoderm. This, together with 

 the median portion of the original amnion, is des- 

 tined to form a vesicle that remains connected 

 by canals with the amnia of the individual em- 

 bryos and is therefore called the common amnion 

 {c am). 



11-^^ 



Fig. 16. — Armadillo egg showing four 

 embryos in early primitive streak stages. 

 (See stage VIII.) (From Patterson, but 

 inverted in order to be comparable with the 

 other stages.) 



