42 THE BIOLOGY OF TWINS 



to leave the part of the embryonic disk that is in contact 

 with the trophoblast and to migrate downward to a 

 position beneath the ectodermal mass. 



Stage III. Gastrulation completed (Fig. lo). — The 

 endoderm cells have now migrated away from the 

 trophoderm and form a complete layer of somewhat 

 flattened, deeply staining cells that lie in close contact 

 with the compact mass of ectoderm cells {ec) on the 

 side away from the trophoblast.^ Very little change 

 occurs in the trophoblast during the first three stages. 

 Eggs of about the stage shown in Fig. 8 are found 

 lightly attached to the uterine wall near the middle of 

 the cross-shaped area shown in Fig. 4. 



Stage IV. Embryonic germ-layer inversion (Fig. 

 11). — This stage is one of the most significant in the 

 entire history in that it shows a curious inversion^ of 

 the normal relations of ectoderm and endoderm. We 

 expect ectoderm to be outside and endoderm to be inside, 

 but in the armadillo a sort of inversion occurs that 

 results in the ectoderm getting inside the endoderm. 

 Although this process is not necessarily followed by 

 twinning, it at least appears to offer a highly favorable 

 opportunity for this type of embryonic doubling. 



Soon after the completion of gastrulation the some- 

 what flattened mass of ectoderm cells begins to round 



^ This method of gastrulation is very strikingly like that described 

 by Hill for the marsupial cat Dasyurus, which is of interest when we recall 

 that the remarkable changes in the ovocyte in this species are also like 

 those of our armadillo. One may be fairly certain that the early cleavage 

 stages of the two species will prove to be similar. 



* A very similar type of germ-layer inversion has been described 

 for several species of rodent. The work of Mellisinos on the mouse is 

 especially interesting in this connection (Arch. mikr. Anat. und Entw., 

 Bd. 70) (1907). 



