26 YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



From the atrial canal the ventricle continues on at the left and runs far forward in the peri- 

 cardial cavity, where it is strongly flexed ventrally and turns caudad as it reaches the median 

 line. It is here attached to the pericardial wall by a short stretch of ventral mesocardium, 

 the only portion of this structure which is still present. Wallin (1913) finds a similar 

 attachment of the ventricle in an embryo of 13 somites. The lower portion of the ventricle 

 runs caudad in the median line ventrally, until it lies under the atrium and end of the sinus 

 venosus, when it turns very sharply dorsally to the left. There is a constriction at this 

 flexure, and the cavity into which the ventricle here opens, which is almost as capacious 

 as the ventricle itself, is the bulbus cordis. The bulbus now turns forward, running on the 

 right side and parallel to the atrioventricular canal and the upper portion of the ventricle. 

 Opposite where the ventricle begins to sink at the approach to its anterior flexure, the 

 bulbus cordis begins to narrow in a funnel-shaped fashion, and at the same time this nar- 

 rowed trunk turns medially and, reaching the middle line, goes suddenly up toward the roof 

 of the pericardial cavity with a little twist backward, and then is immediately flexed 

 cephalad outside the pericardial cavity and directly under the pharynx, in the floor of 

 which it makes a very extensive elevation ; this runs forward a very short distance only as 

 the unpaired ventral aorta, and bifurcates just behind the second branchial pouch. The 

 peculiar loop formed at the end by the bulbus cordis in the embryos of 22 and 23 somites, 

 described by Van den Broek (1911), is not seen here, but it would not take a great deal of 

 displacement or increased flexure of the funnel-shaped portion to produce something like 

 it. The bulbus cordis forms the part of the heart which reaches farthest cephalad and so 

 the heart prominence is much greater and farther forward on the right than the left, as is 

 the case in the Van den Broek embryos and in the one described by Thompson. It will 

 be seen that the heart of this embryo is intermediate between those of stages already 

 mentioned as either just earlier or just older. 



There is a small piece of dorsal mesocardium (plate 3, fig. 4, and plate 4, fig. 2) extend- 

 ing to the roof of the pericardial cavity from the dorsal surface of the bulbus cordis. This 

 membrane is much vacuolated and evidently in process of degeneration. A similar rem- 

 nant of dorsal mesocardium in this location has also been described by Wallin (1913). 



The heart of Embryo V is so similar to that of Embryo VI as to merit no mention 

 except in two particulars. The chambers, flexures, and constrictions of the tube are an 

 exact replica of those already described, but the bulbus cordis projects considerably more 

 cephalad of the rest of the heart than in Embryo VI, and the anterior part of the pericardial 

 cavity for some distance contains only the bulbus cordis and aortic stem. The only other 

 difference is the presence, on the atrium, of a much deeper and more distinct groove dor- 

 sally, so disposed that it does not lie in the median plane, but, starting just in front of tin- 

 left horn of the sinus venosus, runs obliquely toward the middle line, dividing this part of the 

 heart very distinctly, so that to the right of the groove lie the combined sinus venosus and 

 atrium, and to the left only a portion of the atrium. 



The correspondence of the hearts of these two embryos to the heart of embryo Hal 2, 

 modeled by Weintraub, and figured in Keibel and Mall's Human Embryology, is even 

 closer than to the hearts of the Low and Thompson embryos. Of course there is a certain 

 individual difference in general appearance, but the disposition, relations, and sizes of the 

 various chambers and the amount of flexure show a most remarkable similarity. This 

 embryo is 3 nun. long and possesses 15 paired somites, and so is the nearest in general 

 development of all those quoted to the two here described. In all three embryos the 

 ventricle is by far the largest portion of the heart, and the bulbus cordis is next in size. 



