WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 77 



teric veins, and again where they unite in the median line, they are in the interval 

 of a plexiform nature, slightly more open on the right, and dip down on both sides 

 as far as the lowest limit of the myoepicardial mantle. Two small cross anasto- 

 moses are present between these caudal halves of the heart plexus. About its 

 middle and for a short extent the plexus is essentially a single median mass, fairly 

 uniform and irregularly stellate or crescentic in section. Cephalad it broadens out 

 rapidly, becomes more complicated and very markedly plexiform, while its con- 

 stituent elements again assume a roughly symmetrical, bilateral arrangement. At 

 the same time it begins to extend around onto the lateral surface of the pharynx. 

 At the level of the thyroid, high up under the mantle, the cardiac mass breaks up 

 into two roughly symmetrical portions (the right more markedly plexiform) which 

 pass cephalad and dorsad over the ventre (-lateral surfaces of the pharynx and 

 rapidly run out on either side on the roof of the fore-gut near its lateral margins. 

 These structures represent the first two aortic arches and are perfectly definite 

 endothelium-lined channels. They cross the lateral wall of the pharynx some dis- 

 tance in front of what we have called the first entodermic pocket, about midway 

 between this and the buccopharyngeal membrane. At their plexiform origins 

 both arches are sharply limited endothelial channels lying close to the pharyngeal 

 entoderm. From the anterior convexity of the course described by these vessels 

 there is given off a short sprout, the beginning of the internal carotid. Beyond the 

 origin of the carotid the dorsal aortse can be traced for only a very short distance, 

 when they fade out in the aortic grooves on the roof of the fore-gut in the region 

 dorsal to the first pocket. There are no indications of a second arch unless one 

 chooses to put such an interpretation upon certain irregularities found along the 

 lateral border of the plexus in its anterior third and at the base of the first arch. 



Concerning the structure of the heart plexus (text-figure A and fig. 11) a few 

 words are necessary, since the reconstructions give no adequate expression of its 

 essential characters. Only the lateral extremities of the omphalomesenteric veins 

 and the anterior, bifurcated end of the plexus, where the first arches take origin, are 

 genuine endothelium-lined vessels. Between these points the heart is represented by 

 a very irregular, cellular mass, roughly crescentic in section, its concavity directed 

 backward toward the fore-gut, The illustrations do not bring out fully the great 

 irregularity in the surface of this mass, since it has not been possible to reproduce 

 the numberless cell processes with which it is thickly studded. The entire cardiac 

 plexus is extensively vacuolated, the cavities being for the most part separate. 

 Toward the arterial end of the heart these spaces become more frequent, larger, 

 and more often confluent, There is in this stage a freer pathway for diffusion cur- 

 rents outside the plexus, in the wide space underneath the mantle, than there is 

 within the plexus itself, only the two extreme ends of which are pervious. A cer- 

 tain similarity obtains between conditions represented here and those shown by 

 Parker (1915, plate I, fig. 5) in a 15-16 somite embryo of Perameles nasula, but 

 only as regards the general form of the heart, since the vascular development at 

 this stage in Perameles is comparatively very far advanced. In her text-figure 14 

 the heart in cross-section is quite like that of our embryo, except that it presents 



