OVIPAROUS ANIMALS. 



or during their exclusion. In this mode, a place is usual- 

 ly selected where the egg will be exposed to a suitable and 

 uniform temperature, and where a convenient supply of 

 food may be easily obtained for the young animals. Such 

 arrangements prevail in insects. 



In the second, the mother (aided in some cases by the sire) 

 forms a nest, in which she deposits her eggs, and, sitting 

 upon them, aids their hatching by the heat of her body. 

 Birds in general hatch their young in this manner. 



In the last, the eggs are retained in the uterus, without 

 any connection, however, by circulating vessels, until the 

 period when they are ready to be hatched, when egg and 

 young are expelled at the same time. This takes place in 

 some sharks and mollusca. The animals which exercise 

 this last kind of incubation are termed Ovoviviparoiis. In 



trunks are connected with the iliac vessels, and, on account of the thinness 

 of their coats, they afford the best microscopical object for demonstrating 

 the circulation in a warm-blooded animal. 



" The other membrane, the membrana vitetti, is also connected to the body 

 of the chick, but by a twofold union, and in a very different manner from 

 the former. It is joined to the small intestine by means of the ductus vitel- 

 lo-intestinales : and also, by the bloodvessels which have been already men- 

 tioned, with the mesenteric artery and vena portoe. 



" In the course of the incubation, the yolk becomes constantly thinner 

 and paler by the admixture of the inner white. At the same time, innu- 

 merable fringe-like vessels with flocculent extremities, of a most singular 

 and unexampled structure, form on the inner surface of the yolk-bag, oppo- 

 site to the yellow ramified marks above mentioned, and hang into the yolk. 

 There can be no doubt that they have the office of absorbing the yolk, and 

 conveying it into the veins of the yolk-bag ; where it is assimilated to the 

 blood, and applied to the nutrition of the chick. Thus, in the chicken which 

 has just quitted the egg, there is only a remainder of the yolk and its bag 

 to be discovered in the abdomen. These are completely removed in the 

 following weeks, so that the only remaining trace is a kind of cicatrix on the 

 surface of the intestine." BLUMENBACU'S Comparative Atiatomy, London, 

 1807, p. 479-iSi. 



