TWINNING IN DASYPUS NOVEMCINCTUS 55 



Trager nutrition has passed, the Trager rim has dis- 

 appeared, and the egg is beginning to develop ridges 

 which act as physiological drill points and enable the 

 young villi to penetrate the maternal mucosa. A small 

 circular area at the very top of the egg is comparatively 

 free of villous ridges and is destined to become a large 

 non- villous area such as is shown in Fig. 19. 



Each embryo is in a somewhat advanced primitive- 

 streak stage and has formed a broad, bandlike connection 

 with the margin of the developing placenta, a connection 

 that is the forerunner of the umbilicus. A short endo- 

 dermal allantois opens externally and extends inward 

 toward the placental margin, though it is not destined 

 to play any important role in placentation ; it is evidently 

 a vestigial structure. The relation of this structure 

 to the umbiHcus is better shown in the sectional figure 

 of a Uttle later stage (Fig. 26). Each embryo occupies 

 its own extra-embryonic area and is isolated from 

 corresponding areas of adjacent embryos by a sort of 

 partition resembhng the sinus terminahs of an avian 

 embryo. The extra-embryonic areas are covered with 

 blood islands which are the forerunners of the system 

 of vitelUne blood vessels seen in the next figure (Fig. 18). 



Each embryo has its own amnion which is connected 

 by its amniotic connecting canal with the small com- 

 mon amniotic vesicle, whose origin has been already 

 described. 



The two lateral horns seen in the figure are pro- 

 tuberances of the egg membranes that are forced into 

 the openings of the paired Fallopian tubes. These 

 points are very useful as a means of orienting the 

 vesicle with reference to the uterine axes; by means of 



