Development of the Nine-Banded Armadillo. 401 



connecting canals and in one case, in the degree of development 

 of the embryos. 



These observations force us to the conviction that the orienta- 

 tion of the vesicle in the uterus and the pairing of the embryos are 

 expressions of the cleavage polarity and symmetry of the ovum. 

 The cell products of the first two blastomeres would occupy the 

 right and left halves of the early blastocyst and the daughter 

 cells derived from the first two blastomeres would normally 

 hold their relative positions as quadrants of such a blastocyst, so 

 that, although it may not be possible to note any definite demarka- 

 tion of embryonic primordia until a much later stage, they may be 

 well defined from the first. When however pairing seems to exist 

 between diagonally opposed embryos it might conceivably be due 

 to a shifting of blastomeres in the four-cell stage, which could 

 readily occur in such loose cell aggregates as prevail in early mam- 

 malian cleavage stages. A shifting upwards of two diagonally 

 placed blastomeres and a consequent shifting downward of the 

 other two would bring about a recombination of blastomeres 

 into two new pairs without interfering with the hereditary ten- 

 dencies of the individual units. Such an appeal to the imagina- 

 tion of the reader would scarcely be justified were it not the logical 

 outcome of a failure to explain the conditions on any other basis. 

 We are much inclined, in spite of Fernandez' failure to note any 

 indication of a demarkation of separate embryonic areas in his 

 earliest vesicles, to believe that such areas exist from the beginning 

 and express themselves as separate primordia only on the differ- 

 entiation of embryos. This view is in direct opposition to that of 

 Fernandez who holds that up to the time when the separate em- 

 bryos are distinguishable, the vesicle is a single embryo. 



IX. Conditions in Vesicles Containing Five Foetuses 



Out of a total of seventy embryonic vesicles there occurred 

 three in which there were five foetuses. In all of these the sex 

 could be determined and, curiously enough, they were all males. 

 Whether or not this condition is universal could not be determined. 

 If how^ever it should prove that all five-embryo sets are miales it 



JOURNAL OP MORPHOLOGY, VOL. 21, NO. 3. 



