974 SPECIAL PHYSIOLOGY. 



pials, and some Rodents, they combine before they open, forming a 

 double uterus. In the Marsupials, remarkable pouches, the so-called 

 marsupia, supported by special bones, are found in connection with 

 the pelvic organs ; they receive the young, which are born in an im- 

 mature state. In other Rodents, the uterus is cleft. In the Cetacea, 

 Solipeds, and Ruminants, the uterine chamber is double, or two-horned ; 

 in the Carnivora, Insectivora, and Cheiroptera, the horns of the uterus 

 are comparatively short. In the apes, this organ is but slightly notched. 

 The human uterus forms a simple triangular chamber. 



The Heart, Bloodvessels, and Blood. 



The first appearance of bloodvessels in the embryo and its append- 

 ages, can be best observed in the incubated hen's egg. In the middle 

 germinal layer, where it extends over the yolk, linear clusters of nu- 

 cleated cells appear, and arrange themselves in streaks, which soon 

 unite in a retiform manner ; these speedily become distinguished, by 

 special processes of differentiation or development, into an external 

 firmer part, or wall, which forms a future bloodvessel, and an internal 

 softer part, which remains fluid, and further separates into liquor 

 sanguinis and blood corpuscles, some of which are colorless, whilst 

 others soon acquire the characteristic red coloring matter. The fluid 

 central part of these red streaks is blood ; it soon exhibits movement, 

 and, as the vessels become connected, it forms continuous streams. 

 These earliest vessels on the surface of the yolk constitute the so-called 

 vascular area ; they terminate in a marginal bloodvessel, known as the 

 sinus terminally; this, extending beyond the embryo, both in front 

 and behind, divides into vessels which run towards each extremity of 

 the embryo. In the meantime, in front of or beneath the chorda 

 dorsalis, near the cephalic end of the embryo, within the middle ger- 

 minal layer, a solid mass of cells appears, connected with certain 

 branching streaks ; the outer part of this becomes converted into an 

 elongated sac or dilated tube, the primitive heart, and the interior into 

 the liquor sanguinis and blood corpuscles ; whilst from the connected 

 streaks, are developed, an arterial trunk which divides into two 

 branches, at the anterior end of the now tubular heart, and a venous 

 trunk communicating with its hinder end. These rudiments of a pro- 

 pulsive organ or heart, with its embryonic arterial and venous chan- 

 nels, are, moreover, soon connected with the vessels already mentioned 

 as being formed outside the embryo, in the vascular area upon the 

 yolk, and thus a determinate direction is given to the blood circulating 

 through the embryo arid the surrounding vascular area. At this time, 

 there are no capillaries in the embryo ; all the vessels are either ar- 

 terial or venous. The blood appears to pass out from the sides of the 

 embryo, and to reach the terminal sinus, by which it is conveyed back 

 in the direction of the cephalic end of the embryo. The heart, which 

 originates very early after the commencement of the second day in 

 the chick in a group of cells connected with the intestinal portion of 

 the middle blastodermic layer, soon forms a straight longitudinal 

 utricle, or sac, from the anterior end of which a short arterial trunk 



