442 THE PHYSIOLOGY OF REPRODUCTION 



deal, if not the whole, of the space between the amnion and the 

 chorion." 



Oetacea. — There is a diffuse indeciduate placenta in Orca, uniformly 

 studded with branched villi which are absent only at the ends of the 

 chorionic sac, that is, opposite the os uteri and the Fallopian tubes. 

 The sab extends into both cornua. 



SiRENiA. — The placenta in Medicare is diffuse, indeciduate, and 

 zonary (see above, p. 408). The uterine epithehum persists in the 

 crypts. " Stabchen " or hippomanes (see p. 437) have been found. 



Edentata. ^The placenta is said to be zonary and deciduate in 

 Oryderopus (unlike most Edentates), bell-shaped in Myrmecophaga 

 and Tamandua, poly-cotyledonary in ^radypus, oval in Dasypus, and 

 diffuse in Manis and Gholmpus} The details of placental development 

 have not been worked out in any of these animals and the constitution 

 of the embryotrophe is unknown. 



IV. The Placenta in Deciduata 



In the Deciduata three modes of attachment between embryo and 

 mother are found : Centric, in which the blastocyst rests in the cavity 

 of the uterus, attains a large size, and comes in contact with the wall 

 over its whole circumference ; JSxcentric, in which the blastocyst remains 

 small and lodges in a furrow of the uterine mucosa, and later a 

 ! decidua reflexa is formed ; Interstitial, in which the small blastocyst 

 attacks the mucosa at one point and reaches the connective tissue. 

 In this form also a decidua reflexa is forpied. 



In the neighbourhood of the attachment the mucosa degenerates, 

 but the connective tissue cells usually enlarge to form decidual cells 

 before degeneration sets in.^ The capillaries dilate and ^come in 

 contact with the trophoblast. The mucosa interlocks so closely with 

 the foetal villi that the two tissues cannot be separated without injury. 



Caenivoea. — The Carnivora are characterised by a zonary form 

 of deciduate placenta. The foUowiug account of its development 

 refers particularly to the dog and cat, which have been most 

 frequently investigated. The gestation period in the cat is on an 

 average sixty days, and in the dog fifty-eight to sixty-three days. In 

 both the ovum takes a comparatively long time to traverse the 

 oviduct. On reaching the uterus the blastocyst is covered by a thick 

 prochorion which prevents adhesion for a considerable period. 



The mucosa is matured, as in Ungulates, at the first prooestrum 

 by the development of a well-differentiated sub-epithelial cellular 

 layer, and of the glands and crypts (see p. 430). The crypts provide 



^ Jenkinson, Vertebrate Embryology, Oxford, 1913. 

 2 But see p. 457. 



