282 STUDY OF YOUNG CHICK EMBRYOS. 



passes over on to the somatopleure, Som, and so on to the amnion, Am. On the 

 ventral side of the pharynx the mesothelial layer passes over into the muscular 

 wall of the heart, m. hi. The heart itself is very large; it has two tubes, the en- 

 dothelial, endo, and the muscular, m. ht, which are very distinct. The endothe- 

 lial cavity is very large. It is especially expanded immediately underneath the 

 pharynx to form the auricular end of the heart, which receives the veins. 

 Throughout a large part of the auricle the endothelium is closely fitted against 

 the muscular wall. Further ventral wards the double heart-tube bends to the 

 right of the embryo to form the ventricular limb, Ven, in which the endothelial 

 cavity is also enlarged. The heart as a whole occupies about one-half the area of 

 the entire section of the embryo, being of relatively enormous proportions. The 

 cardinal veins, D. C, have moved down as compared with the previous section, 

 and are now found to lie in the somatopleure, in which there also appear several 

 sections of smaller blood spaces above the main cardinal vessel. The path of the 

 cardinal through the somatopleure carries it toward the heart. The vertical 

 part of the vessel, which affects a union with the heart, is known as the duct of 

 Cuvier. The ducts of Cuvier also deliver the blood from the posterior cat dinals 

 to the heart. They are at somewhat different levels on the two sides of the em- 

 bryo, that on the right side being lower and occupying a sort of prominence on 

 the mesothelial side of the somatopleure. If the cardinal veins are traced along 

 through successive sections, it will be found that they open directly into the auri- 

 cles of the heart, crossing over the coelom, Cos. The crossing is accomplished by 

 a growth of the somatopleure which unites with the wall of the heart. The 

 openings of these veins are at this stage morphologically symmetrical and are 

 entirely distinct from the openings of the omphalo-mesaraic veins, which enter 

 the heart further tailwards. If sections in the series between the present one and 

 that through the aortic end of the heart (Fig. 157) be examined, it will be found 

 that the heart in the middle part of its course is entirely detached from the 

 pharynx, so that the heart-tube is suspended by its two ends from the ventral 

 side of the pharynx. By the crossing of the cardinal veins the portion of the 

 coelom, Cos, on either side of the pharynx is shut off from the portion of the coelom 

 around the heart. At the raphe, raph, of the amnion the ectoderm of the amniori 

 joins that of the chorion, Cho. In the portion of the somatopleure, Am' , which 

 runs from the raphe to the embryo there are a number of spaces of rounded form 

 which appear like so many vesicles. The nature of these vesicles is uncertain.* 



* They seem to be bounded on one side by ectoderm, on the other by mesoderm ; but as to this, and as 

 to the significance of these vesicles, I cannot express any opinion. The separate opening of the ducts of Cuvier 

 in front of, and independently of, the omphalo-mesaraic veins, so far as I am aware, has not been" hitherto 

 recorded. It is a condition of morphological importance. 



