THEORIES OF POLYEMBRYONIC DEVELOI'MKXT 89 



pendently of the others, except in so far as devclo])- 

 ment within a common chorion and the necessity of 

 sharing a single primary placenta involve mutual 

 adjustments. 



Various other theories purporting to olTer explana- 

 tions of the production of plural embryos from single 

 eggs have been advanced, but the three that have been 

 most persistently maintained are the ''blastotomy 

 theory," the "budding theory," and the ''fission theory." 



BLASTOTOMY VERSUS BUDDING 



Is each embryo the lineal descendant of a single 

 blastomere of the four-cell stage of cleavage or does 

 the process of fission heretofore described occur with- 

 out reference to the cell products of the early cleavage 

 cells ? About these contrasting theories of polyem- 

 bryony in the armadillo there has been much differ- 

 ence of opinion and some shifting of viewpoints on 

 the part of individual workers. In one of their earlier 

 papers (1910) Newman and Patterson stated that it 

 seems highly probable that the tissues involved in each 

 of the four quadrants of an embryonic vesicle do really 

 arise as the lineal descendants of one of the first four 

 blastomeres. Still earlier in 1909 they said: ''In the 

 case of Dasypus each embryo probably arises from one 

 of the blastomeres of the four-celled stage.'' It may 

 be said that, so far as the writer is concerned, no idea 

 of blastotomy in the sense that there was any actual 

 physical separation or isolation of blastomeres was ever 

 entertained. Patterson, however, in iqt ^ in discussing 

 the various theories of polyembryony, classes the 

 theory "that each embryo is the Hneal descendant of 



