OVIPAROUS ANIMALS. 491 
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 im 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 Ovoviviparous. 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 vitellz, 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 porte. 
‘¢ 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 nto 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.’— BLumensacu’s Comparative Anatomy, Loadon, 
1807, p. 479-484. 
