POLYEMBRYONIC DEVELOPMENT IN TATUSIA 633 



In the armadillo the facts of development which we have pre- 

 sented in the descriptive part of this paper are fully in accord 

 with the theory of budding. We have seen that the early phases 

 of differentiation are similar to those of other mammals in which 

 the ovum produces but one embryo. Prior to the appearance of 

 the embryos, the ectoderm, the entoderm, and the exocoelomic 

 mesoderm are differentiated, and later all three of these layers are 

 concerned in the formation of the embryonic buds. The initial 

 step in budding apparently occurs in the embryonic ectoderm, but 

 the entodermal layer is soon involved in the process. It is there- 

 fore entirely correct to say that the seat of budding in the arma- 

 dillo is to be found in the blastoderm, that is, the blastoderm i$ 

 the budding organ. It may be possible to extend this same con- 

 ception to accidental or sporadic cases of polyembryony occurring 

 in the lower forms which lay yolk-laden eggs. 



The most important point brought out in this study is the fact 

 that polyembryonic development in the armadillo can be inter- 

 preted as a type of budding; and, while to show that polyembry- 

 ony is a budding process does not solve the question as to the 

 determining cause of the division of the blastoderm, yet it is a 

 distinct step toward the solution of that important problem. 



It is perhaps premature to attempt an explanation of the ulti- 

 mate cause of polyembryony. We first need a comprehensive 

 study of each of the forms in which it occurs. Such investiga- 

 tions, followed by well-directed experiments, may yield results 

 that will reveal at least some of the factors which control poly- 

 embryonic development. At present only a few suggestions 

 need be made; and, first, we may briefly consider the ideas that 

 have been expressed by some of those who have worked on the 

 subject. 



Harmer ('93), in his excellent paper on embryonic fission in 

 Bryozoa, points out several interesting comparisons that can be 

 drawn between the process of multiple-embryo formation in 

 Crisia and budding in many other organisms. He calls atten- 

 tion to the fact that, in at least some of these forms, embryonic 

 fission is connected with the deviation from the normal type of 



