Mat 25, 188S.] 



SCIENCE. 



451 



These observations were made under various 

 conditions with no more than usual care, and 

 probably represent fairly the accuracy easily 

 attainable. With further practice the errors 

 could probablj' be reduced. In general, my 

 observations seem to show that single sets 

 of observations bj- this wedge-photometer are 

 trustworthy to one or two tentlis of a magni- 

 tude. If so, there is much that can be done 

 bj- it ; and as the simplicity, convenience, and 

 inexpensiveness of the instrument are such 

 as to recommend it, similar instruments could 

 properl}- be a part of the outfit of everj' obser- 

 vator3-. 



The above errors are correct on the sup- 

 position that none of the stars examined were 

 variable ; and I found no evidence that they 

 were. In the case of another star, however, 

 either the star was variable, or the errors made 

 were much larger than in the other cases, 

 though the observations were made at about 

 the same time. The star in question is 

 22°. 2162. The average difference between it 

 and 22°. 21 64 is 25.1 seconds for twenty-three 

 sets ; but the individual sets range from 28.0 

 seconds on the loth, at 13h. sidereal time, to 

 22.3 seconds on the 19th, at 12h. The mean 

 error of a single set is 1.34 seconds, and the 

 probable error of the mean, ±0.58 second. 

 As I believed I could trace with the eye a 

 change in the brightness of the star, I think 

 we have in this case a variable, with a range 

 of about one magnitude, rather than obser- 

 vations much less accurate than others taken 

 at the same time. M. W. Harrington. 



NOTES UPON THE FOETAL MEM- 

 BRANES OF THE OPOSSUM AND 

 OTHER MARSUPIALS. 



I RECENTLY had the good fortune to receive 

 from Mr. Robert Speir of South Orange, N.J., 

 a female opossum which had been captured 

 within a few daj's after impregnation. I was 

 thus enabled to make very satisfactory obser- 

 vations upon the foetal membranes, about 

 which there has been so much uncertainty for 

 many j-ears. These embryos were in an early 

 stage of growth, and, although they plainly 

 showed verj- novel and unexpected features, no 

 positive conclusions could be reached as to 

 their later development. At this point a cor- 

 respondence with Professor Wilder of Cornell 

 resulted in his very generously sending me a 

 quantity of marsupial material which he had 

 procured from Australia. Among this mate- 

 rial was a nearl}- perfect foetus in a late stage 

 of development. An examination of this fully 



confirmed the observations upon the opossum 

 embryos, and showed the relations of the foetal 

 mcmljranes at a later period. More recently 

 Professor Chapman, of the Jefferson medical 

 college, has kindly allowed a thorough exami- 

 nation of a valuable kangaroo foetus in his 

 possession, which he has described in the 

 proceedings of the Philadelphia academj' for 

 1881. This foetus was in a stage intermediate 

 between that represented b\' the opossum em- 

 bryos and that of the foetus sent me bj' Pro- 

 fessor Wilder : it showed the same features as 

 the other specimens in an intermediate stage 

 of growth. 



In all these specimens the membranes are 

 arranged ver}' much as those of a kangaroo 

 foetus which Professor Owen described in 

 1833. The peculiarity of the foetal mem- 

 branes of this animal, which has ever since 

 been used as a basis of classification distin- 

 guishing the marsupials from the higher mam- 

 mals, is, that the allantois never attains a very 

 great size, so that nothing like an allantoic 

 placenta is formed ; and the function of absorb- 

 ing the maternal nutrition, during the short 

 period of intra-uterine life, has alwaj-s been 

 considered to have devolved entirely upon the 

 yolk-sac. Professor Owen, in the older of 

 the specimens which he examined, found that 

 the membranes were arraiiged as follows : ' the 

 foetus was enveloijed in a large subzonal mem- 

 brane, with folds fitting into uterine furrows, 

 but not adhering to the uterus, and without villi; 

 the embrj-o was enveloped in an amnion re- 

 fleeted over the stalk of the j'olk-sac. This sac 

 was large and vascular, and was connected with 

 the foetal vascular S3'stem hy a vitelline artery 

 and two veins. There was a small allantois sup- 

 plied by two allantoic arteries and one vein : 

 it was quite free, and not attached to the sub- 

 zonal membrane. The area of attachment of 

 the j'olk-sac to the inner surface of the subzonal 

 membrane formed a disk bounded by the sinus 

 terminalis, or circular venous trunk. When 

 spread out, therefore, the j-olk-sac formed the 

 figure of a cone, of which the apex was the 

 umbilical cord, and the base the sinus ter- 

 minalis. 



These valuable observations were confirmed 

 'by Professor Chapman in his paper referred to 

 above. Thej' are accurate so far as they go ; 

 but they leave us in doubt as to the real rela- 

 .tions which exist between the foetus and the 

 mother, inasmuch as they give no clew to the 

 manner in which the embrj-o is nourished dur- 

 ing its intra-uterine life, — a period of about 



^ This description is largely taken from Balfour's Comparative 

 embryology, vol. ii. p. 199. 



