EARLY STAGES OF THE DEVELOPMENT OF THE FOWL. 929 



the embryo projects; the latter is the amniotic sac, and is subsequently filled 

 by the amniotic fluid. The allantois (Fig. 333, c) is not formed till the 2d 

 or 3d day, when it appears as a bud thrown out by the splanchnopleure 



FIG. 330. 



FIG. 331. 



FIG. 330. Plan of early Uterine Ovum. Within the external ring, or zona pellucida, are the serons 

 lamina, a; the yolk, 6; and the incipient embryo, c. 



FIG. 331. Diagram of Ovum at the commencement of the formation of the Amniou : a, chorion; 6, 

 yolk-sac ; c, embryo; d and e, folds of the serous layer rising up to form the amnion. 



close to its junction with the Somatopleure at the hinder end of the embryo, 

 and gradually insinuating itself first into the pleuroperitoueal cavity, and 

 then between the true and false amuiotic sacs, it curves over the embryo 



FIG. 333. 



FIG. 332. Fecundated Egg of Fowl with Allantois nearjy completed : 1, Inner lamina of amniotic 

 fold; b, outer lamina of the same; c, point where the amniotic folds come in contact with each other; 

 the allautois is seen penetrating between the inner and outer laminae of the amniotic fold. 



FIG. 333. Fecundated Egg, showing formation of Amnios and Allantois: a, Umbilical vesicle; b, am- 

 niotic cavity ; c, allantois. 



(Fig. 332), and is separated from the outer air only by the shell vitelline 

 membrane and the thin false arnnion. It is composed of a layer of columnar 

 cells derived from the hypoblast, of a layer of cells derived from the undi- 

 vided mesoblast, and of an epithelial investment derived from the mesoblast 

 of the splanchnopleure. 1 It receives at an early period branches from the iliac 

 arteries, which subsequently constitute the umbilical vessels, and are developed 

 in an extension of the mesoblast cells of the area opaca, known as the vas- 

 cular area. 



The first rudiment of the heart in the chick appears as a solid thicken- 

 ing of the mesoblast of the splanchnopleure just beneath the anterior ex- 

 tremity of the foregut. It soon becomes hollow, and receives behind the 

 blood of the two omphalo-mesaraic veins, which ramify over the yolk-sac, 



1 Dobrynin, Wien. Akad. Sitz-ber., Ixiv, 1871, Abth. ii. 



