328 ivan e. wallin 



mesoder:\ial structure 



There are thirteen pairs of mesodermal somites. These are 

 hardly discernible on the surface. The first pair is situated at 

 a level of about midway between the neck flexure and the hind- 

 brain flexure. The last pair is opposite the poiAt where the 

 allantois leaves the hind-gut. A myocoele may be observed in 

 most of the somites. The cells of the somite are arranged in 

 a radial manner. 



The pleuro-peritoneal coelom communicates with the extra- 

 embryonic coelom on the two sides of the yolk stalk. In its 

 cephalic portion it communicates with the pericardial coelom. 

 The lateral lips bounding the open part of the pleuro-peritoneal 

 coelom have a thickened edge produced by the allantoic veins 

 which 11U1 cephalad in this position. 



The septum transversum is present as a single layer composed 

 of the cephalic wall of the yolk stalk fused with the caudal part 

 of the pericardium. 



The excretory system is represented by pronephric tubules. 

 The morphological details of these, as far as I have studied them, 

 agree with the description given by Fehx ('12). 



VASCULAR SYSTEM 



The heart tube is composed of three parts, bulbus cordis, ven- 

 tricle and atrium. The atrium is situated in the mid-line of 

 the body immediately cephalad to the septum transversum. Its 

 greatest diameter is transverse. From the left extremity of the 

 atrium the atrial canal runs to the left, ventral and cephalad 

 to the ventricle. The ventricle pursues a course from the left 

 to right, ventrally and somewhat caudad. At the right extremity 

 of the ventricle the heart tube makes a sharp bend so that its. 

 continuation, the bulbus cordis, comes to lie parallel with the 

 ventricle. The bulbus cordis has a fairly constant size up to 

 its cephalic end where it diminishes slightly. It ends in the 

 mid-line of the body. 



The ventral wall of the cephalic end of the bulbus cordis is 

 continuous with the pericardium as is also the case with the 



