452 



SCIENCE. 



[Vol. I., No. 16. 



seventeen daj-s in the opossum/ and thirtj-- 

 eight daj-s in the kangaroo.^ My fortunate 

 discovery of the early opossum embryos, and 

 the subsequent examination of the two other 

 marsupials, seem to throw a great deal of light 

 upon this question, if they do not actually 

 solve it. The principal facts which have been 

 brought out may be briefl3^ stated. 



1. In the opossum the j'olk-sac spreads 

 out over about one-third of the inner area of 

 the subzonal membranes, and forms a highly 

 vascular disk, the false chorion of the placen- 

 tal mammals. This disk is ventral to the 

 embryo ; and among the numerous embryos 

 which were examined in situ, these disks were 

 found to be invariably placed in a long ute- 

 rine furrow, while the remainder of the envel- 

 oping membrane floated free in the cavity of 

 the uterus. The use of the word ' attachment' 

 would be misleading in this connection, as a 

 slight touch with the needle was sufficient to 

 remove the embrj'os from their position. The 

 outer surface of the subzonal membrane, all 

 over the area to which the j'olk-sac was adher- 

 ent, was found to be covered with minute villi, 

 which were just visible to the naked eye . These 

 villi are simple upgrowths of the subzonal epi- 

 thelium, shaped like little hillocks, and con- 

 fined to this area. At this early stage they 

 are hollow. 



2. In Professor Wilder's specimen,^ villi 

 were found to be scattered over the same area 

 of subzonal membrane ; but in this case their 

 development had proceeded much farther, and, 

 although they were extremely minute, each 

 was found to be provided with a solid papilla, 

 which arose from the epithelium of the 3"olk- 

 sac. A closer examination showed that the 

 cap of subzonal epithelium was composed of 

 flattened cells, and that the papilla was pro- 

 vided with capillar}' branches derived from the 

 vessels of the yolk-sac. These villi conform, 

 therefore, to what Professor Turner has de- 

 scribed as the simplest type of allantoic vilH, 

 the nearest approach to which, among the pla- 

 cental mammals, is found in the pig. 



3. In the kangaroo foetus the villi could be 

 seen without a lens. They were, however, so 

 minute, that it is not at all surprising that they 

 have been overlooked hitherto. Thej- were 

 spread over the highly' vascular portion of the 

 yolk-sac, which is loosely attached to the" sub- 

 zonal membrane. A close examination into 

 their structure has not vet been made. 



4. The allantois in the opossum embrj'os 

 was found in various stages of growth, but in 

 none was it attached to the subzonal mem- 

 brane. In Professor Wilder's specimen it was 

 highly vascular, and appeared to show a disk- 

 like area of attachment to the subzonal mem- 

 brane. This area showed no traces of villi. 

 The subzonal epithelium consisted of flattened 

 cells. In the kangaroo it was an extremely 

 small vascular sac. 



5. Owing to an accident, one horn of the 

 uterus in which the embryos were preserved 

 in situ was destroj'ed, so that no satisfactorj' 

 study of the uterine wall could be made. 



The presence of villi over that portion of the 

 subzonal membrane which is in contact with 

 the uterine wall renders it highly probable from 

 analogy that minute crj'pts are present upon 

 the latter. At all events, we now have data 

 sufficient to establish the following facts : that 

 the so-called false chorion of some of the lower 

 orders of placental mammals, formed b}' the 

 spreading of the yolk-sac over the inner sur- 

 face of the subzonal membrane, in the marsu- 

 pials functions as a true chorion, developing 

 simple villi, by which the maternal and foetal 

 blood-vessels establish a feeble interchange : 

 in other words, the functions of the allantois 

 in the placental mammals are, in a rudimentary 

 v/ay, performed by the yolk-sac in the marsu- 

 pials. Finally, some genera of the marsupials 

 probably show the attachment of the allantois 

 to the subzonal membrane, which is the first 

 step towards the establishment of an allantoic 

 placenta. 



These facts naturally give rise to a number 

 of interesting questions, which will be discussed 

 in a paper to be published in the Quarterly 

 journal of microscopical science for July. 



I wish to express m^' indebtedness to Pro- 

 fessors Wilder and Chapman, without whose 

 aid these observations would have been very 

 incomplete. Henry F. Osboen. 



Morphological laboratory, 

 Princeton, May 11, 1SS3. 



1 See Bachman, Proc. acad. nat. sciences Phllad., 1848, 44. 



2 See Owen, Comp. anat. and pliys. of the vertebrates, ill. §400. 

 * The genus cannot be ascertained, owing to a misplaced 



label. The foetus undoubtedly belonged to one of the smaller 

 Australian genera. 



RAINFALL AT PANAMA. 



In the Comptes rendus for Feb. 26, M. de Lesseps 

 publishes some interesting observations of rainfall 

 for four years (1879-82) at the Isthmus of Panama. 

 The accompanying table gives these observations, to- 

 gether with like observations at stations along the 

 Pacific coast, which are added for the ptirpose of 

 comparison. 



M. de Lesseps remarks that the rainy season lasts 

 about six months, from May to November, with an in- 

 terruption at the end of June and beginning of July. 

 He assigns as a cause for these peculiarities the ad- 

 vance of the (overhanging) sheet of rising air which 



