470 PROCEEDINGS OF TUE ACADEMY OF [1881. 



condition of its blood-vessels taken in connection with the length 

 of the embryo and the short time that the latter remains in tlie 

 uterus, makes it impossible for me to think that in the Kangaroo 

 a placenta is ever developed. I vise the word placenta in the 

 sense ordinarily accepted, meaning a structure which consists of 

 the interlacing of the allantoic blood vessels with those of the 

 decidua serotina of the uterus, that is, of that part of the hyper- 

 trophied mucous membrane of the uterus in contact with the 

 ovum. Further, that while the umbilical vessel is fused through 

 part of this surface with the chorion, the chorion is only in con- 

 tact with the inner surface of the uterus, not adhering to it in 

 an}' way. The disposition of these membranes in the Kangaroo 

 embryo is therefore difterent 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 foital Kangaroo, such nutriment must 

 osmose through the omphalo-mesenteric vessels. The contact of 

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

 character. The embryo Kangaroo itself measured six-eighths 

 of an inch in length from the mouth to the root of the tail. 

 The latter was one-eighth of an inch long. The mouth was 

 open, and the tongue, though large, was not protruded. The 

 palpebral folds were not developed. There was no sign of an 

 auricle. Four branchial clefts could be distinguished. The 

 anterior extremities were well developed, but the digits had not 

 appeared. The posterior extremities, were represented onlj' 

 by small buds, not very apparent except with a lens. In- 

 dications of the ribs were distinctly visible. The membranous 

 spinal cord could be seen ; the elements of the vertebrae 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 



