490 THE PHYSIOLOGY OF REPRODUCTION 



uterine surface and fuses with it. In this way the completed placenta 

 is discoid (Gohre^). 



Peimates. — The order of the Primates includes monkeys, apes, 

 and man. Hubrecht and Jenkinson also include Tarsius, a lemur 

 (see p. 440). Owing to the difficulties of securing material for 

 investigation, many details regarding the early stages of development 

 of the foetal membranes and placenta are yet unknown. 



From the researches of Turner, it is known that the placentation 

 is in general the same throughout the order, except for differences in 

 the size and form of the villi, and in the structure of the decidua. 

 On the other hand, the Primates are distinguished from all other 

 placental Mammals in that they .do not form an allantoic placenta. 

 Notwithstanding the variations in the degree of its development, in 

 all the orders previously considered the allantois projected free into 

 the extra-embryonic coelom before it was united with the wall of the 

 blastodermic vesicle. In the Primates and Tarsivs the embryo is 

 attached from the beginning to the wall of the blastocyst by the 

 " Bauchstiel " or " Haftstiel," a mesodermal connecting-stalk first 

 observed by His^ in human embryos. The allantois appears very 

 early as a recess of the posterior wall of the yolk-sac before the 

 formation of the hind-gut. It never projects free into the coelom, 

 but is contained as a narrow tube in the " Bauchstiel " without 

 reaching at any time the wall of the blastocyst (Fig. 147). The 

 trophoblast is in this way vascularised dir6otly, and a cJwrionic 

 instead of an allantoic placenta is formed. For this reason Hubrecht 

 has suggested that the term chorion should be restricted to the 

 Primates. Minot^ strongly supported the views of His. He wei^t 

 even further, and stated that the placenta was also chorionic in 

 Carnivora, Eodentia, Insectivora, and Cheiroptera, but his views have 

 not beeii generally accepted. Regarding the modification in Primates, 



'■ Gohre, " Dottersaok und Placenta des Kalong {Ftergpus edidis)," Studien 

 liber Entiuickhmgsgeschichte der Thiere, Selenka, vol. v., 1892. The formation of 

 the amnion in bats has recently been investigated by Da Costa (" Sur la forma- 

 tion de I'amnios chez les Ch6iroptferes {Miniopterus schreibersii) et, en g^nSral, ' 

 chez les Mammifferes," Mem. Soc. Fortugaise des ,Sci. Nat., Porto, 1920). He 

 describes three phases : (1) The appearance of a closed cavity in the embryonic 

 disc, the upper wall of which is the primordial amnion ; (2) the rupture of this 

 wall and the disappearance of the primordial amniotic cavity, which is replaced 

 by a tropho-ectoblastic space ; (3) the formation of an amnion and a definitive 

 amniotic cavity with ectoblastic folds. In some other bats, Murinus and the 

 Noctule, it is the same, except that the primordial amniotic cavity is less definite. 

 Primordial amniogenesis of the same type is seen in the guinea-pig, Galeopithecus 

 Tatvsia, and in Primates. The primitive cavity disappears and is replaced by 

 a cavity which is limited by the amniotic folds in Mici'ocheiroptera, the pig, 

 mouse, etc. The primitive cavity may be absent or only hinted at, as in the 

 hedgehog, mole, rabbit, etc., Tarsius, Carnivora (see above, p. 412). 



^ His, Anatomie menschlicher Emhi-yonen, I. 



^ Minot, Human Embryologi/, Boston, 1892. 



