1887.] NATURAL SCIENCES OF nilLADELPIirA. 117 



The speaker then contrasted the various types of placentation, 

 pointing out that it was largely a matter of how the blastodermic 

 vesicle was primarily brought into relatioii with the walls of the 

 uterus. If the foetation occurred in the bicorned or tulnilar type of 

 uterus there was an obvious tendency toward the diffuse or zonary 

 type of placenta, as shown in Ungulates and Carnivora, and in those 

 uniparous forms in which the foetus occupies mainly one horn of the 

 uterus, and in which there are bare poles to the chorion and a 

 bare spot Avhere the latter comes in contact with the os uterus as in 

 the Mare and Dugong. Here, the mere mechanical relations of 

 the foetal and maternal surfaces obviously had had an influence in 

 determining the form of the placenta. The zonary placenta was 

 also imitated in Arthropods {Perijmtus) in virtue of the existence 

 of such similar conditions in both the latter and Carnivorous 

 Mammalia. The speaker thought that all attempts to use the pla- 

 centa as a means of clearly distinguishing the various orders of 

 mammalia or of subdividing the latter into sub-classes would, in the 

 course of further embryological research, be shown to be not well 

 founded. This seemed all the more probable since the rationale of 

 the so-called " inversion " of the germinal rays of Rodentia was 

 better understood. As a result of fuller knowledge it is hardly 

 conceivable that a zonary placenta could be found in those types, 

 notwithstanding the fact that they at first seem to present the same 

 type of condition for the blastodermic vesicle in the uterine 

 cornua as do the Carnivora. But now that we know that some of 

 the Insectivora (Talpa) tend in the same direction the anomalies 

 which are presented by Insectivora and Rodentia become explain- 

 able and lead us up to the vicAV that, it depends (1) upon the mode 

 in which the early development is modified, and (2) upon the man- 

 ner in which the foetus is related to the maternal surfaces, whether 

 the diffuse, zonary or discoidal form of placenta will be assumed. 



In the case of the Sloths and Ant-eaters, of South America, the 

 uterus has attained a remarkable degree of specialization, so as to 

 greatly resemble the simple uterus of the higher Primates, and in 

 this case again, the relationship between the form of the uterine 

 cavity and that of the placenta seems obvious, for in the sloths, 

 ant-eaters and higher primates, the placenta is essentially discoidal 

 and deciduate. In the sloth, however. Turner has sho^v*ll the 

 discoidal placenta to be made up of separable lobes ; these may be 

 conceived as representing the cotyledons of Ungulates, or groups of 

 tufts in the diffuse type of placenta, which have been crowded 

 together as the uterine cornua became shortened on the mesometric 

 side, in the transition from the bifid to the simple type of uterus. 

 Some further ground for this view of the origin of the lobulated 

 discoidal, dome-shaped placenta of the sloths is supplied by the fact 

 that in Manis, or the scaly Ant-eater of the Old World, the placenta 

 is diffuse and non-deciduate. In some of the Armadilloes the 

 placenta is transversely oblong, and this again is a fact favorable to 

 the preceding view. 



