480 EMBEYOLOGY OF THE LOWER VERTEBRATES oh. 



The eggs are retained in the ovary, either in the follicle, or in 

 the cavity of the ovary ; more rarely in the dilated oviduct or uterus. 

 The developing embryo may depend ^for its nourishment upon the 

 yolk (Scorpaenidae) ; it may absorb nourishment by the surface of 

 the yolk-sac which grows out into villi (Anableps) ; or the nutritive 

 secretion of the ovarian wall may be taken into the alimentary 

 canal and there digested (Embiotocidae). 



Among the Amphibia true viviparity is rare. A well-marked 

 case occurs in Salamantlra atra (Wiedersheim, 1890). Here a large 

 number (40-60) of eggs pass into the oviduct when breeding is about 

 to take place but of these all except the one (in rare cases as many 

 as four) next the eloacal opening simply break down forming a kind 

 of broth which fills the oviducal cavity. The embryo nourishes 

 itself, after it has used up its own yolk supply, by gulping down and 

 digesting this fluid, which contains not merely the yolky debris of 

 disintegrated eggs, but also large quantities of red blood corpuscles 

 derived from extensive haemorrhages of the uterine wall. 



Perhaps the most striking feature of the Mammalia is the 

 extreme degree of adaptation which they typically show to an 

 intra-uterine mode of development in which the embryo leads a 

 parasitic existence attached to the uterine lining of the mother. In 

 accordance with this the external ectoderm of the blastocyst becomes 

 modified to form organs of attachment which eventually, in the 

 region of 'the yolk-sac and more especially in the region -of the 

 allantois, become vascularized and elaborated into the complex 

 nutritive and respiratory organs named placentae. This being so, 

 it becomes of much interest to enquire whether amongst those 

 Amnio ta which are lowest in the scale of evolution — the Reptiles — 

 there are any foreshadowings of the type of adaptation to intra-uterine 

 development found in the Mammalia. Probably numerous such 

 cases exist but at the present time, with our extremely imperfect 

 knowledge of Reptilian development, we are acquainted with only 

 a few. The most interesting of these is that of the Italian Lizard 

 Chalcides tridactyhcs (Seps chalcides). Giacomini's description of 

 this (1891) may be said to form the foundation of what will one day 

 probably form an important chapter in Vertebrate embryology. 



The eggs, which measure about 3 mm. in diameter, are first found 

 in the oviducts early in May, while the first young are born towards 

 the end of July, the period of gestation thus being between one and 

 two months. 1 The eggs become spaced out along the oviduct or 

 uterus, so as to give it a moniliform appearance, each egg being 

 arranged with its apical pole towards the mesometrium. At about 

 the middle of gestation the "egg" presents the appearance shown 

 in Fig. 219, A, the whole forming a kind of blastocyst about 7 mm. 

 in diameter. The outer surface is formed by the ectoderm of the 

 serous membrane. Within the serous membrane there can be seen 

 the allantois with transparent, richly vascular, wall and the yolk- 



1 About sixty-five days according to Miugazzini. 



