fluid moving in the fetal area is colorless at first, even 

 when the heart is already contracting. Only after pulsation 

 begins is it possible to see the movement of red blood from 

 the vascular area. Baer, however, was somewhat doubtful as 

 to the accuracy of his observations. He was unsure whether 

 he could talk about the movement of blood in canals which 

 are still devoid of walls and therefore provide no clearly 

 outlined route (I 2r, p. 69 (34)). 



In the rudiment of the heart, the anterior canal grows 

 into two thin branches, which disappear without reaching the 

 roof of the digestive sac. The pulsation of the heart still 

 pushes a colorless blood anteriorly, as in the heart of 

 insects. In this period the heart is located directly under 

 the future head and from both sides is enclosed by the 

 anterior parts of the abdominal plates. Later on, it 

 stretches forward from these plates and protrudes downwards. 

 The canals coming out from the anterior end of the heart, 

 Baer indicated, joint later, forming the beginning of the 

 aorta. The heart becomes curved by the middle of the second 

 day; by the end of this day it curves downwards and anteriorly 

 even more, so that its ends come close together. Three pairs 

 of arterial arches form, one after the other. Nearly in the 

 middle of the embryonic body at that time, the entry point 

 of the veins appears. 



The initial condition of the other parts of the vascular 

 system Baer described as follows. At the border of the 

 vascular region, a blood receptacle forms as two semi-circles, 

 at first without defined walls. This is the blood circle, 

 which embryologists later called the border vein. Blood 

 enters the middle of each semi-circular sinus and moves 

 anteriorly and posteriorly. Anteriorly from the sinus go 

 many vessels, flowing into a common stem which reaches the 

 embryo (frequently there are two such stems) . Vessels from 

 the posterior part of the sinus merge into an ascending vein, 

 which with the anterior vein pours into the left heart 

 protrusion. The blood goes into the heart anteriorly and, 

 passing through the stem, it goes out through three pairs 

 of arterial arches which curve upwards and pour into two 

 stems. These merge together at first, then they divide again 

 along the way to the posterior end of the embryo. In the 

 middle part of the body, they form two thick branches which 

 reach to the border sinus. 



318 



