THE AMNION. 



85 



vertebrae have begun to be formed, an appearance of a chorda in their 

 centre is perceptible. The two laminse viscerales or ventrales, which, as 

 described by Von Baer, are also formed in the serous fold, and proceed 

 from the laminae dorsales, continue at first on the same plane with the ger- 

 minal membrane, and only by degrees bend downwards and inwards 

 towards the cavity of the yolk, where they unite and form the anterior 

 walls of the trunk. While the borders of the primitive groove are 

 approaching each other and about to close, the groove itself at its anterior 

 extremity dilates into three recesses or vesicles (fig. 11, B), which, with the 

 nervous matter developed in them, constitute the rudimentary brain ; at 

 the same time, the surrounding parts assume the characters of the head 

 of the embryo which raises itself above the surface and bends forwards 

 and downwards as in the chick. During the development of the above 

 parts of the embryo, an accumulation of cells takes place between the two 

 laminse of the germinal mem- 

 brane at the area vasculosa. Fig. 12.* 

 These cells shortly form them- 

 selves, as in the bird, into a dis- 

 tinct layer the vascular lamina 

 which serves as a ground- 

 work in which the first blood- 

 vessels of the embryo are deve- 

 loped. The mode in which this 

 development of vessels takes 

 place will be described hereafter. 



Amnion. The development 

 of the amnion, which, as was 

 pointed out by Von Baer, is 

 effected in Mammalia just as 

 in birds, takes place very ra- 

 pidly, being completed by the 

 end of twenty-four hours after 

 the first appearance of the pri- 

 mitive groove, f 



Umbilical vesicle. Having remarked on the existence of an umbilical 



end presents three dilatations, B, which correspond to the three divisions or vesicles of the 

 brain. At its lower extremity the groove presents a lancet-shaped dilatation (sinus rhomboi- 

 dalis) C. The margins of the groove consist of clear pellucid nerve-substance. Along the 

 bottom of the groove is observed a faint streak, which is probably the chorda dorsalis. D. 

 Vertebral plates. After Bischoff. (Ibid.) 



* Fig. 12. Embryo from a bitch at the 23d or 24th day, magnified ten diameters. It 

 shews the net- work of blood-vessels in the vascular lamina of the germinal membrane and 

 the trunks of the omphalo-mesenteric veins entering the lower part of the S-shaped heart. 

 The first part of the aorta is also seen. After Bischoff. (Ibid.) 



t Bischoff, Entwick. der Saugeth. und des Menschen, p. 108. 



