1640 A MANUAL OF ANATOMY 



known as hysterotomy, the following structures are met with, in 

 order from without inwards: 



1. TDecidua vera. 



2. J. Decidua capsularis or reflexa. 



3. (Chorion leve. 



4. Amnion. 



5. Amniotic cavity and liquor amnil. 



6. Fcetus. 



The Vascular System. 



The vascular system consists of (i) the heart, (2) the bloodvessels, 

 and (3) the blood, all of which are developed from the mesoderm. 

 The heart is developed from the embryonic mesoderm, whilst the 

 primitive bloodvessels and the blood are derived from the extra- 

 embryonic mesoderm. The earliest form assumed by the vascular 

 system is associated with the vitelline circulation. This gives place 

 to the allantoic circulation, and this, in turn, is supplanted by the 

 placental circulation. 



Bloodvessels. — ^The earliest bloodvessels, called vitelline or 

 omphalo-mesenteric, are four in number — two arteries and two veins. 

 They are developed in connection with the wall of the yolk-sac, 

 which consists of splanchnopleure — ^that is to say, splanchnic meso- 

 derm and entoderm. The region of the yolk-sac in which they 

 appear is called the vascular area, which is disposed in the form of 

 a band or strip surrounding the embryo. 



The first stage consists in the formation of cells, called angioblasts 

 or vessel-formers, which are arranged as cords or cylinders, known 

 as cell-cords. These cell-cords are disposed in a reticular manner, 

 the reticulum being called the angioblastic reticulum, and within its 

 meshes there are groups of cells. 



The second stage consists in the flattening and extension of thr 

 superficial cells of the cell-cords to form continuous endothelial 

 walls, which represent the lining walls of the future bloodvessels, 

 and which enclose the remainder of the cell-cords. 



The primitive bloodvessels, therefore, are at first solid cords or 

 cylinders, each representing a cell-cord, surrounded by an endo- 

 thelial wall. 



The third stage consists in the hollowing out of these solid cell- 

 cords, so that each now acquires a lumen. Each contained cell- 

 cord becomes permeated by a clear fluid, which has entered from 

 the surrounding mesoderm, and the cells are pressed towards the 

 walls of the vessels. In these situations they form groups, called 

 the blood-islands of Pander, which are nests or ' cradles ' of primitive 

 red corpuscles. These groups encroach more or less upon the 

 lumen of the vessel, and may even temporarily occlude it. The 

 individual cells of each group increase in number by mitotic or 

 karyokinetic division, and, entering the blood-stream, they repre- 

 sent the first corpuscles of the embryonic blood, which are called 

 erythroblasts. The blood-islands, being thus gradually deprived 



