496 ox THE ZOOLOGICAL POSITION 



mammals. In Ungulates the cavity of the allantois is very large 

 and its outer wall is in conta,ct with the inner surface of the 

 chorion over very nearly its whole extent, the yolk-sac becoming 

 very small. The chorion grows out into villi over its whole 

 surface, and these villi penetrate into pits or crypts of the iniier 

 surface of the uterus, without becoming united with uterine 

 tissue. The mesoblast of the allantois with its blood-vessels 

 grows into these villi and thus is formed the difi\ised placenta. 



In Man the amnion appears to be formed as a closed cavity, 

 and the hypoblast develops as described above, but instead of 

 extending round the inner surface of the chorionic vesicle it 

 forms a small closed sac, and meso blast develops on the inner 

 surface of the chorion and the outer surface of the yolk-sac with 

 a large cavity between, which is the extra-embryonic coeloni. 

 There is at first no mesoblast within the embryo, but it extends 

 outside the amnion and attaches the embiyo to the mesoblastic 

 lining of the chorion. As the embryo develops, this connection 

 lengthens somewhat and becomes first posterior then ventral. 

 This connecting mesoblast is the umbilical cord, and rejiresents 

 a solid allantois, into which a very rudimentary cavity extends 

 from the hinder end of the gut. The mesoblast at the outer 

 end of the stalk forms villi over a disc-shaped area of the 

 chorion and so forms a discoid placenta. 



With regaixl to these peculiarities of development, the ordinary 

 Lemurs agree with the Ungulata and Tai^sius agrees with Man. 

 It may be said, therefore, that there is as much difference between 

 the development of Tarsius and that of a Lemur as there is 

 between that of Man and that of an Ungulate. Moreover, in 

 Man the trophoblast destroys and absorbs the uterine mucous 

 membrane and the allantoic villi extend into embryonic tro- 

 phoblast-tissue. In this respect also Tarsius agrees with Man 

 and differs from Lemurs. To suppose that the mode of develop- 

 ment of Tarsius and Man has been independently evolved 

 without any close relationship between the two seems to me 

 imreasonable. 



The development of Tarsius is more similar to that of Man in 

 some respects than that of Monkej^s and Apes. In many 

 Monkeys there are two placentas, a dorsal and a ventral ; in Man 

 only one, which is dorsal. In Hylohates and Simla there is only 

 one placenta, but it is ventral. In Tarsius the placenta is 

 single and dorsal. The Apes, however, are nearer to Man in 

 having the chorion entirely imbedded in uterine tissue, which 

 is not the ease in Monkeys or Tarsius. 



Dr. P. Chalmers Mitchell, F.R.S. : — Characters have to be 

 judged as well as counted, if it be intended to use them for 

 estimating the relative degree of affinity between animal types. 

 No anatomist doubts but that Man retains many primitive 

 characters ; Anthropoid Apes, Old-World Monkeys, American 

 Monkej^s, Tarsius, and Lemurs also retain many primitive 



