482 EMBRYOLOGY OF THE LOWER VERTEBRATES m. 



to the actual placental projections for even the smooth parts of the 

 surface over the allantoic area are provided with an extraordinarily 

 rich network of capillaries which show an even more marked 

 tendency than those of the uterus to penetrate into the epithelium. 

 Over the smooth area the foetal and maternal surfaces are in 

 intimate contact, so that the two capillary networks lie parallel and 

 close to one another, separated only by two very thin epithelial 

 membranes. In the region where foetal and maternal projections 

 interlock chinks are apparent between the two in which there 

 appear to be traces of a fluid material probably nutritive and 

 secreted by the maternal epithelium which as already mentioned has 

 in this region a glandular appearance. 



The yolk-sac placenta is less highly developed. In the region of 

 the centre of the yolk-sac flattened ridge-like projections also appear 

 which interlock with corresponding uterine projections and become 

 vascularized as the mesoderm spreads beneath them. Between the two 

 surfaces is the remnant of vitelline membrane but this, gradually disap- 

 pears so that foetal and maternal surfaces come into intimate contact. 



Chalcides (Gongylus) ocellatus, another Italian lizard, is also 

 viviparous and in it occur similar though less marked adaptations to 

 viviparity (Giacomini, 1906). Here in the later stages of gestation 

 the general arrangement of the foetal envelopes resembles that in 

 C. tridactylus. The allantoic region of the foetal surface is smooth 

 and possesses a rich capillary network. It lies in immediate contact 

 with the uterine lining, which in this region is covered with very 

 thin flattened epithelium overlying an extremely rich network of 

 maternal capillaries. 



The portion of uterine lining in relation with the vitelline region 

 of the foetal surface is less richly vascular, is covered with thicker 

 epithelium of vacuolated cells with large nuclei, and is thrown into 

 low folds which interlock with corresponding folds of the foetal 

 surface so as to form an incipient yolk-sac placenta. The foetal 

 epithelium of this region is thickened and in places columnar and 

 appears t have an absorbent function. As in C. tridactylus remains 

 of membrane are to be seen for a time between the foetal and 

 maternal surfaces in this region. 



To sum up, we find in Chalcides ocellatus a less advanced sta^e 

 of adaptation to intra-uterine development than in C. tridactylus 

 1'robably similar conditions will be found in various other viviparous 

 Li/ards as r.ij. in tin- Australian. Trachysaurus and Tiliqua scincoii/f* 

 (Cyclodu* l,u<l<l'icrti) (Haacke, 1885). 



In tli.- PdimUwoim (Anguis frayilis) and in the Viper ( /'// 

 and Smooth snake ifforonella austriaca) viviparity also occurs but 

 here in a .-till more definitely incipient form a thin shell per>isi in<_: 

 throughout development and the lortal envelopes and uterine lining 

 ivmainiii'j- practicallx unmodified. 



Thus in the three -els of LVplilo above mentioned we see three 

 steps in the evolution of \i\iparity: 



