FCETAL NUTRITION: THE PLACENTA 463 



mass is pressed against the uterine surface and fuses with it. 

 In this way the completed placenta is discoid (Gohre ^). 



Primates. — The order of the Primates includes monkeys, 

 apes, and Man. Hubrecht also includes Tarsius, a lemur (see 

 p. 410). 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 placenta- 

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

 differences in the size and form of the viUi, and in the structure 

 of the decidua. On the other hand, the Primates are distin- 

 guished from aU 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 tlie wall of the blastodermic 

 vesicle. In the Primates and Tarsius the embryo is attached 

 from the beginning to the wall of the blastocyst by the " Bauch- 

 stiel " or " Haftstiel," a mesodermal connecting-stalk first ob- 

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

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

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

 ccelom, but is contained as a narrow tube in the " Bauchstiel " 

 without reaching at any time the wall of the blastocyst (Fig. 120). 

 The trophoblast is in this way vascularised directly, and a 

 chorionic 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 went even further, and stated that the placenta was 

 also chorionic in Carnivora, Rodentia, Insectivora, and Cheirop- 

 tera, but his views have not been generally accepted. Re- 

 garding the modification in Primates, Hubrecht * says : " Once 



^ Gohre, " Dottersack und Placenta des Kalong (Pteropus edulis)," 

 Studien iiber Entwicklungsgeachichte der Thiere, Selenka, vol. v., 1892. 



' His, Anatomie menschlicher Embryonen, I. 



' Minot, Human Embryology, Boston, 1892. 



i See Robinson's " Hunterian Lectures," Journ. of Anat. and Phya., 

 vol. xxxviii., 1904. 



