POLYEMBRYONIC DEVELOPMENT IN TATUSIA 599 







answer to this question we are at once confronted with the much 

 more important problem of the origin of the multiple embryos 

 from the single fertilized egg. In the paper cited above the 

 position was taken that the embryos belonging to a pair were 

 probably derived from one of the blastomeres of the two-celled 

 stage, and that each embryo could therefore be looked upon as 

 a lineal descendent of one of the blastomeres of the four-celled 

 stage. In the present contribution the view is held that the 

 four embryos do not owe their origin to a spontaneous blas- 

 totomy, but rather that they are the product of a form of 

 agamogenesis belonging to the general category of budding. 



It was considered then of the utmost importance to determine 

 at just what point in the development of the armadillo blasto- 

 cyst evidence of its quadruplicity first appeared. Consequently 

 a sharp lookout was maintained in the study of all the early 

 stages for signs of the first expression of polyembryony. The 

 earliest observed evidence which could be interpreted as repre- 

 senting the beginning of multiple embryos comes in the forma- 

 tion of the mesothelium not in the manner in which the ele- 

 ments of this layer arise, for localized centers of proliferation 

 were not found, but in the early formation of the two large 

 mesodermal vesicles through the fusion of several smaller ones. 

 The development of two mesodermal vesicles would not in it- 

 self be so significant, as it might be merely an expression of a 

 bilateral arrangement of mesoderm similar to that of many other 

 vertebrate embryos, were it not for the fact that they hold a 

 position corresponding exactly to the two primary ectoderma.1 

 buds; that is, they lie on the sides of the vesicle which are di- 

 rected towards the openings of the fallopian tubes. However, 

 it may be that these two mesothelial vesicles have no general 

 significance with reference to polyembryonic development, for it 

 must be kept in mind that they have arisen by the fusion of 

 numerous smaller vesicles, and later they in turn fuse to form a 

 single vesicle. 



Whatever may be the significance of the position of the two 

 mesodermal vesicles, certain it is that the first indisputable evi- 

 dence of the differentiation of the four embryos from the blasto- 



