Placenta

For further details see Quain's Anatomy, vol. i. (London, 1908) ; and, for literature, 0. Hertwig's Handbuch der Entwickel ungslelire (Jena).

Comparative Anatomy.

If the placenta is to be regarded as a close union between the vascular system of the parent and embryo, the condition may be found scattered throughout the phylum of the Chordata. In so lowly a member as Salpa, a placenta is formed, and the embryo is nourished within the body of its parent. In some of the viviparous sharks, e.g., the blue shark (Cacharias), the yolk-sac has ridges which fit into grooves in the wall of the oviduct and allow interchange between mater nal and foetal blood. This is an example of an "umbilical pla centa." In the viviparous blennies (Zoarces viviparus), among the teleostean fishes, 200 or 30o young are nourished in the hol low ovary, which develops villi secreting nutritive material.

The mammals are divided into Placentalia and Aplacentalia; in the latter group, to which the monotremes and most marsupials belong, the ova have a great deal of yolk, and the young, born in birth are simply withdrawn, the decidua being left behind in the uterus, so that these placentae are spoken of as non-deciduate while other kinds are deciduate.

For further details

see S. W. W. Turner, Lectures on the Comparative Anatomy of the Placenta (Edinburgh, 1876) ; A. Robinson, "Mam

malian Ova and the Formation of the Placenta," Journ. Anat. and Phys. (i904) xxxviii., 186, 325. For literature up to 1906, R. Wieders heim's Comparative Anatomy of Vertebrates, trans. and adapted. by W. N. Parker (London, 1907) ; 0. Terasaki, Ueber die Githerfaser structuren i. d. menschlichen Placenta, Ztschr. f. Geburtsh. u. Gyn., Qs, 94 (1927, bib') ; J. W. Williams, Placenta circumvallata, Am. J. Obst. and Gyn., 13, I (1927, bibl.) ; H. Westermark, Weight of the Human Placenta relative to that of the Foetus, etc., Acta Obst. et Gyn. Scandin., 4, 249 (1925-26, bibl.), and F. L. Adair and H. Thelander, Am. J. Obst. and Gyn., io, 172 (1925, bibl.) ; H. Richter, Zur Physiol ogie u. Morphologie der Placenta, Arch. f. Gyn., 124, 557 (1925, bibl.) ; J. E. Davis, B. V. Kellog and A. L. Amolsch, Anatomical and Clinical Studies upon 875 placentae, Am. J. Obst. and Gyn., 7, 637 (1924, bibl.) ; G. I. Strachan, Physiology of the placenta, J. Obst. and Gyn., Brit. Emp., 32, 89 (1925, bibl.) ; 0. Grosser, Vergleichende u. mensch liche Placentationslehre, in Halban and Seitz, Biologie u. Pathologie d. Weibes, vol. vi., part 1, p. 41 (bibl.) ; H. A. Dietrich, Biologie der Placenta, ibid. p. 195 (bibl.) ; H. Hinselman, Normales u. path. Verhalten der Placenta u. d. Fruchtwassers, ibid. p. 241 (bibl.).