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THE AMEBIC AN NATURALIST [Vol.XLVII 



these villous regions expand they come into contact at 

 their margins and apparently fuse into a lobate zone, 

 which had been called a compound zonary placenta. 

 Finally the zone separates along the dorsal and ventral 

 lines to form two lateral notched discoid placentae, to 

 which we need scarcely apply a name. It is obvious that 

 there is nothing to be gained by attempting to classify 

 such a placental complex or by comparing it with those of 

 other groups of mammals, for the peculiar conditions 

 seen here are obviously merely very special adjustment 

 to the peculiar conditions arising from polyembryonic 

 development within a single chorion. The foetuses after 

 they have once been separately outlined are distinct, com- 

 plete units and are associated scarcely more closely than 

 are the embryos of other forms of mammals where sev- 

 eral individuals develop simultaneously in a single uterus, 

 for they have their own separate amnia and separate 

 placentation, and there is absolutely no admixture of 

 foetal blood. 



Without further burdening the reader with an elabora- 

 tion of embryonic details and relations we may briefly sum- 

 marize the situation in-so-far as the question of specific 

 polyembryony is involved. The ovogenesis is normal; 

 a single egg is fertilized by a single spermatozoon; the 

 cleavage is apparently normal and gives rise to a blasto- 

 dermic vesicle similar to that of other mammals, espe- 

 cially the rodents ; germ-layer-inversion affords an easy 

 mechanism for producing several embryos in a single 

 chorion, for the quadruplets arise by means of dichot- 

 omous budding of the inner ectodermic vesicle without 

 affecting the enveloping membranes of the vesicle, which 

 form the common chorion; the subsequent embryonic 

 development of the several embryos is as independent as 

 it can be under monochorial conditions, since each indi- 

 vidual has its own separate amnion, allantois, umbilicus 

 and placenta. This in brief is the polyembryonic situa- 

 tion, a consideration of which offers for solution several 

 problems peculiar to the material. What are the physio- 



