EMBRYO WITH TWENTY -EIGHT SEGMENTS. 207 



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 

 effects a union with the heart, is known as the common cardinal. The common 

 cardinals also deliver the blood from the posterior cardinals to the heart. They are 

 at somewhat different levels on the two sides of the embryo, 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 sec- 

 tions, it will 'be found that they open directly into the auricles of the heart, cross- 

 ing over the ccelom, Cce. The crossing is accomplished by a growth of the somat- 

 opleure which unites with the r wall of the heart. The openings of these veins are 

 at this stage morphologically symmetrical and are entirely distinct from the open- 

 ings of the omphalo-mesaraic veins, which enter the heart farther tailward. If 

 sections in the series between the present one and that through the aortic end of 

 the heart (Fig. 155) 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, Cce, 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 amnion joins that of the chorion, Cho. In the 

 portion of the somatopleure, Am', which runs from the raphe to the embryo there 

 area number of spaces of rounded form which appear like so many vesicle?. The 

 nature of these vesicles is uncertain.* 



The secondary somites, My, are very characteristic, and should" be studied 

 with a higher power. The somite consists of an outer layer and an inner 1,: 

 "of about equal thickness, and these two layers pass over into one another at the 

 dorsal and ventral edges of the segment. They are closely pressed against one 

 another, so that there is no space between them. The outer layer is more deeply 

 stained than the inner; its nuclei are somewhat less distinct and are rounded in 

 form. Those of the inner layer are elongated in form, as may be easily observed 

 by raising and lowering the focus. The outer layer is quite close to the ectoderm, 

 and the inner layer rests against the large mass of mesenchymal tissue which sur- 

 rounds the spinal cord, notochord, and aorta. 



Section through the Anlage of the Liver (Fig. 157). In this section the general 

 topography is similar to that of the last, so that we need describe only the new 



* They seem to be bounded on one side by ectoderm, on the other by mi-,oderm; but as 

 the significance of tht;se vesicles, I cannot express any opinion. The separaii- < '^mn.sj?^' 

 front of, and independently of, the omphalo-mesaraic veins, so far as I 

 It is a con- ; ological importance. 



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