98 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



In connection with the development of the heart, figure 1 of plate 5 and 

 figure 1 of plate 1 are very interesting. The specimen from which the former 

 was taken was one of a litter of five, all of which were injected. It measures 

 6 mm. in oil, that is, after fixation and dehydration, and has 20 somites. The 

 specimen on plate 1, figure 1, was one of a litter of six embryos, all of which were 

 injected. It measured 7 mm. when fresh and is 6.2 mm. long in oil. It has 23 

 somites. It should be noted that these embryos do not have a caudal flexure, 

 so that these measurements must not be confused with the same measurements 

 of older specimens after the flexure has formed. The number of somites gives 

 more valuable data in regard to the stage of development in these stages than do 

 measurements. 



If comparisons are made with human embryos at the stage of 23 somites it 

 will be noted that at this stage the human embryo has two very marked flexures, 

 shown, for example, in the R. Meyer embryo No. 300, represented in Felix's 

 figure 531, in the "Manual of Human Embryology" (Keibel and Mall), and hence 

 it is very much shorter. 



In figure 1 of plate 5 the changes in the heart from the stage shown in plate 4, 

 figure 3, are readily followed. The direction of the ventricular arch has changed 

 from the horizontal to an oblique position. The atrio-ventricular canal has 

 become the characteristic long, slender channel, and there is a marked constriction 

 between the ventricle and the large bulbus cordis. The fretum Halleri is now a 

 long, slender tube, and both the bulbus cordis and the truncus arteriosus are 

 shown in maximum distension. 



In plate 1, figure 1, the form of the sinus venosus is not clear, as it is con- 

 cealed by the injection of the sinusoids of the liver. In all of the six specimens 

 of this litter the sinusoids of the liver are farther developed on the left side than 

 on the right. In all of the other specimens, however, and on the right side of 

 this specimen, there is a marked constriction between the liver and the sinus 

 venosus just below the upper large opening of the umbilical vein. At this stage 

 the umbilical vein connects with the liver and with the sinus venosus by large 

 openings, and with the duct of Cuvier by an extensive capillary plexus in the 

 somatopleure. There is a constriction between the sinus venosus and the atrium, 

 and a well-marked atrio-ventricular canal. The bulbo-ventricular cleft gives the 

 effect of an hour-glass constriction of the heart. This is true of all the specimens 

 of the litter, but in one the contraction of the bulbar portion is particularly marked. 

 The differences in the form of the heart in figure 1, plate 5, and figure 1, plate 1, 

 are partly due to the fact that the hearts in these specimens were fixed while 

 beating and were caught at different phases of the beat. For example, in plate 5, 

 figure 1, and bulbus cordis and the truncus arteriosus show a maximum distension, 

 while in plate 1, figure 1, the bulbus cordis and the truncus arteriosus are con- 

 tracted and there is a general distension of the cephalic aorta. On the other 

 hand, in plate 1, figure 1, is shown the beginning of a torsion of the ventricular 

 loop, by means of which the beginning of the fretum Halleri will come to be 

 opposite the ventricular end of the atrio-ventricular canal. 



