THE SOMITES OF THE CHICK 87 



mental vessels are present. The aorta lies directly beneath the 

 somite, but it is almost in contact with the lateral plate. The 

 posterior cardinal vein lies directly above the Wolffian duct which 

 in turn lies in a groove between the lateral plate and the nephro- 

 tomic mesenchyma. 



The twenty-fifth somite of each side in embryos of thirty-three 

 segments (H. E. C. nos. 97 and 1587) is triangular. The aortae 

 have moved together and have fused. The entire nephrotome 

 with the posterior cardinal vein has moved toward the median 

 plane, and consequently the sclerotome is now bounded ventrally 

 by the aorta, the mesonephric tubules, the Wolffian duct, and the 

 posterior cardinal vein. The dorsal surface of the somite, owing 

 to the lateral inclination downward of the dorsal lamella, makes 

 an angle of 45° with the sagittal plane. 



The lateral edge of the dorsal lamella has grown ventro-later- 

 ally some distance beyond the ectodermal ridge which formerly 

 marked the lateral boundary of the somite and now is pushing 

 into the dense mesenchyma of the Wolffian ridge. The myotome 

 now underlies all of the dorsal lamella but a small area in the middle 

 of its ventro-lateral edge. The sclerotome is definitely divided 

 into a loose medial and a dense lateral part. The two sclero- 

 tomes of this segment have united in the median plane so that a 

 nearly continuous sheet of sclerotomic tissue separates the aorta 

 and the notochord. Neural crest cells continue to migrate into 

 the spinal ganglion. 



After the first phase of rapid development, changes in the struc- 

 ture of the twenty-fifth segment are less rapid. The dermomyo- 

 tome of each of the twenty-fifth somites of an embryo of fortj^-four 

 segments is essentially as far advanced as that of the second somite 

 of an embrj^'o of forty segments. There is, however, a striking 

 difference between the two (compare fig. 16 with fig. 9). The 

 ventral edge of the dermomyotome of the twenty-fifth somite 

 contains a center of growth and extends downward until it comes 

 in contact with the coelomic epithelium. This edge of the 

 dermomyotome thus comes to lie medial to a large wedge-shaped 

 process of the mesenchyma of the Wolffian ridge. This process 

 extends upward between the dermomyotome and the ectoderm to 



