442 THE PHYSIOLOGY OF REPRODUCTION 



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

 clmrion." 



CETACEA. There is a diffuse indeciduate placenta in 0/r. 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 sac extends into both cornua. 



SIKKNIA. The placenta in Halicore is diffuse, indeciduate, and 

 xonary (see above, p. 408). The uterine epithelium persists in the 

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



KUKNTATA. The placenta is said to be zonary and deciduate in 

 Uri/i-fero/ntx (unlike most Edentates), bell-shaped in Myrmecophaga 

 and TV ma ml iia, poly-cotyledonary in Bradypus, oval in Dasypu.x, and 

 dill'use in .}f<i,/i.-< and Choke}} us. 1 The details of placental development 

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

 of the embryotrophe is unknown. 



I\'. THE PLACENTA IN DEQIDUATA 



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 ; Ewentric, in which the blastocyst remains 

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

 decidua retiexa 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 formed. 



In the neighbourhood of the attachment the mucosa degenerates, 

 but the connective tissue cells usually enlarge to form decidual cells 

 lefore 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. 



CARNIVOUA. The ('arnivora are characterised by a aonary form 

 of deciduate placenta. The following 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 prooastrura 

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

 layer, and of the glands and crypts (see p. 4MO). The crypts provide 



1 Jenkinson, Vert^n-nto Kmhn/ology, Oxford, 1913. 

 - But see p. 4">7. 



