A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. Hi! 



medial, and lateral, are all of about equal thickness. They are epithelial in char- 

 acter and contain one to two layers of cells. In the myoccele are found a few 

 scattered stellate cells not unlike mesenchymal cells; later these will enter into 

 the formation of the sclerotome. Mitotic figures are numerous among the cells 

 of the walls and the myoccele, but those of the walls are always at the upper ends 

 of the cells, i. c., the ends bordering on the myoccele. This somite corresponds 

 quite closely with the first coccygeal somite of the Ingalls embryo, except that it 

 probably has fewer cells in its cavity. 



The nineteenth somite (ninth thoracic) shows a somewhat more advanced 

 condition. The myoccele in its lower portion is entirely filled with cells. The 

 ventral half of the medial and all of the ventral wall are breaking up. The cells 

 of these walls, together with those on the inside of the myoccele, form the sclerotome. 

 These cells have pushed out slightly toward the chorda dorsalis, forming the 

 notochordal process. The somite corresponds with the sacral somites of Ingalls's 

 embryo. 



The fourteenth somite (fourth thoracic) is more distally located from the 

 median plane than the previously described somite. Its ventral and medial walls 

 have both broken down and lost their epithelial character and appear as a mass of 

 mesenchyma between the remainder of the somite laterally, the medullary tube 

 and chorda medially, and the dorsal aorta, coelomic epithelium, and posterior 

 cardinal vein ventrally. The notochordal and aortic processes of the sclerotome, 

 lying dorsally and laterally to the dorsal aorta respectively, are easily recognized. 

 The lateral wall is somewhat thicker than that of the above-described somite, 

 being composed of apparently two layers of distinct columnar cells. The dorsal 

 edge of this wall is bent first medially, then ventrally, and comes to lie near the 

 median surface of the lateral wall. It is, however, separated from the lateral 

 wall by a cleft-like portion of the myoccele. The dorsal border of this cleft 

 that is, the groove formed by the rolling over of the medial wall has been termed 

 by Williams 49 the "upper myotomic groove." At the place where the bent-over 

 portion of the dorsal border of the median wall is in contact with the sclerotome 

 it has left a groove on the medial surface of the somite. This has been called 

 the "lower myotomic groove" by Williams and others. The ventral edge of the 

 lateral wall is also turned in medially but to a lesser degree. The myoccele, which 

 as stated before is cleft-like at its dorsal part, is larger and broader ventrally. 

 Owing to the breaking-down of the medial wall, a wide opening is left in it, the 

 so-called intervertebral cleft. This somite, on the whole, is quite similar to Ingalls's 

 lumbar somites. 



The twelfth somite (second thoracic, plate 4, fig. 2), has its sclerotomic cells 

 scattered between the remainder of the somite laterally and the medullary tube 

 and chorda medially. The dorsal edge of its lateral wall has folded over and grown 

 ventrally along the medial surface of this wall, and has united with the turned- 

 up ventral edge except at one place. The lateral wall can now be described as 

 being composed of an outer lamella (cutis plate, dermatome) and an inner lamella 

 (muscular plate or myotome). The intervertebral cleft which lies at the caudal 



