58 THE BIOLOGY OF TWINS 



is ever established. The common amnion and the 

 amniotic connecting canals still persist and serve by 

 their forked arrangement to indicate the pairing of 

 embryos, for the amniotic canals of the two embryos 

 derived from one side usually unite into a short single 

 canal just before they enter the common amnion. 



Stage XI. A middle-aged egg showing four discoid 

 placentae (Fig. 19). — In order to show the embryos and 

 their placental connections in a vesicle of this stage of 

 development, it is necessary to remove a part of the 

 vesicle wall together with parts of the placenta. The 

 drawing represents an egg with a large window cut out 

 from the median ventral wall. Dotted hues indicate 

 those parts of the two placental disks which have been 

 removed; nothing else of any consequence has been 

 removed. The entire egg measures about 35 mm. long 

 and 30 mm. wide; each fetus has a head-rump length 

 of about 14 mm. 



The placenta consists of four separate ovoid disks 

 of treelike villi. The disks of paired fetuses are closely 

 appressed but visibly independent, while there is a 

 distinct space both dorsally and ventrally between the 

 placental disks. 



A large area occupying the whole upper part of the 

 egg has become essentially non-placental, although 

 sparse villi are seen dotted over its surface, especially 

 near the margins of the specialized placental areas. 



The four fetuses are seen to be clearly paired, one 

 pair facing toward the right and the other toward 

 the left; there is much more space between the 

 umbilical attachments of unpaired than between paired 

 individuals. 



