340 Miscellaneous. 



vessels, taken in connexion with the length of the embryo and the 

 short time that the latter remains in the uterus, makes it impossible 

 for me to think that in the Kangaroo a placenta is ever developed. 

 I use the word placenta in the sense ordinarilj- accepted, meaning 

 a structure which consists of the interlacing of the allantoic blood- 

 vessels with those of the decidua serotiaa of the uterus— that is, of 

 that part of the hypertrophied mucous membrane of the uterus 

 in contact with the ovum. Further, while the umbilical vessel is 

 fused through part of this surface with the chorion, the chorion 

 is onlv in contact with the inner surface of the uterus, not adhering 

 to it in any way. The disposition of these membranes in the Kan- 

 garoo embryo is therefore different from the so-called placenta of 

 certain Sharks, which consists in the interlacing of the omphalo- 

 mesenteric blood-vessels with those of the uterus. This structure 

 in the Sharks, though called a placenta, is not homologous with 

 the mammalian placenta, this consisting, as we have seen, of the 

 allantoic vessels and those of the uterus. The Kangaroo cannot 

 be said, therefore, to have a placenta in either sense in which that 

 word is used. The small size of the embryo Kangaroo at birth 

 would lead me to suppose that it drew its nourishment from the 

 umbilical vesicle like the reptile or bird, rather than from the 

 uterine walls as in the mammal. If the uterus does contribute 

 to the nourishment of the foetal Kangaroo, such nutriment must 

 osmose through the omphalo-mcsenteric vessels. The contact of 

 the chorion with the uterus, however, is of a very adventitious 

 character. The embryo Kangaroo itself measured six eighths of an 

 inch in length from the mouth to the root of the tail. The Litter 

 was one eighth of an inch long. The mouth was open ; and the 

 tongue, though large, was not protruded. Ihe palpebral folds 

 were not developed. There was no sign of an auricle. Four bran- 

 chial clefts could be distinguished. The anterior extremities were 

 Avell developed ; but the digits had not a])pcared. The posterior 

 extremities were represented only by small buds, not very apparent 

 except with a lens. Indications of the ribs were distinctly visible. 

 The membranous spinal cord could be seen, the elements of the 

 vertebree being as yet ununited. A penis was visible just in front 

 of the anus. On the supposition that the theory of evolution is 

 true, one would naturally expect to find forms intermediate in their 

 structure and development between the reptiles and birds on the 

 one hand and the placental mammalia on the other. As is well 

 known, in the structure of its skeleton and generative apparatus, 

 the Or^nithorhi/nchxs resembles very closely the reptile and bird, 

 while, as we have just seen, the foetal membranes of the Kangaroo 

 recall the corresponding parts in the reptilian-bird type and fore- 

 shadow those of the placental mammal. If the parts in question 

 have been truthfully described and correctly interpreted as partly 

 bridging over the gap between the non-placental and placental 

 vertebrates, they supply exactly what the theory of evolution 

 demands, and furnish, therefore, one more proof of the truth of 

 that doctrine. — Froc. Acad. Nat. Sci. Philad., Dec. 27, 1881. 



