TWINNING IN DASYPUS NOVEMCINCTUS 49 



bilaterality of the uterus, for we can see no other way 

 of explaining the coincidence that exists between the 

 uterine and embryonic axes. No important change 

 has occurred in the Trager ring except that further 

 invasion of the uterine mucosa has continued. In 

 some eggs the disk of the trophoblast {tr d) lying within 

 the Trager ring appears to be quite free from the 

 uterine mucosa and to form the boundary of a more or 

 less extensive fluid-filled cavity, which had been called 

 by Fernandez the Trager cavity. This cavity is 

 evidently of little morphological significance and may 

 be ignored in subsequent stages. 



Stage VII. The origin of quadruplets. Secondary 

 embryos formed (Fig. 14). — It is at the stage shown in 

 Fig. 14 that the second step in twinning occurs, but 

 the figure, because it is a bilateral sectional view of the 

 egg, fails to show the secondary embryos. The primary 

 embryos II and IV lie respectively to the right and to 

 the left of the egg, while a shorter secondary embryonic 

 outgrowth appears to the left side of each primary 

 embryo, so that the two secondary embryos lie with 

 their axes pointed one toward the dorsal and the other 

 toward the ventral side of the uterus. The embryo on 

 the dorsal side is called III and is said to be the second- 

 ary embryo paired with the primary embryo IV, while 

 the ventral secondary embryo is called I and is similarly 

 related to the primary embryo II. The outline sketch 

 (Fig. 15) shows the axes of the four embryos, seen from 

 the distal end of a vesicle like that shown in Fig. 14. 

 Note that there are evidences of tertiary outgrowths 

 between I and IV and between II and III. In Dasypus 

 hybridus such outgrowths evidently form embryos, for 



