YOUNG TWIN HUMAN EMBRYOS WITH 17-10 PAIRED SOMITES. 15 



Williams the upper myotomic groove, becomes more and more a rounded angle (text fig. 2), 

 until beyond the thirteenth somite it is impossible to state that it is present. The lower 

 myotomic groove is distinct as far back as the thirteenth segment, but after that it is hardly 

 recognizable, although the distinction between myotome and sclerotome is clearly shown 

 by the character and arrangement of the cells and by a slight chink, or split, in the wall 

 at the point where this groove will develop. 



The sclerotome is present in every somite, though in the first and last it is only dis- 

 tinguishable from the rest of the somite by the slightly looser arrangement of the tissue and 

 more irregular outline. These two somites also are the only two possessing a well-marked 

 core, which in each case fills up and almost obliterates the somitic cavity. There is evidence 

 of a core in the other somites, but it has evidently fused with the sclerotome and passed 

 ventrally with the latter, largely losing its identity. 



The notochordal process of the sclerotome (text figs. 1 and 2) is present in all segment s 

 from the second to the sixteenth, and in this series shows a marked decrease in development 

 in each segment proceeding caudad. It comes off the whole ventral extent of the sclero- 

 tome and curves inward under the medullary canal to the notochord, but nowhere does it 

 surround the latter, merely coming laterally into contact with it. As far back as the 

 seventh segment the notochordal processes have fused and obliterated the lower portion 

 of the intersegmental clefts originally present between them. These clefts still persist 

 between every two processes posterior to this point. 



The aortic process is present in each sclerotome (text fig. 1) as far back as the thir- 

 teenth, where it finally disappears. No two of these fuse or come into contact except in 

 somites 2 and 3, and they are separated by intervals considerably larger than the inter- 

 segmental clefts, as they are developed, not from the whole ventral extent of the sclerotome, 

 but only from the anterior three-fourths to two-thirds of this extent. Beyond the thir- 

 teenth segment there is no evidence whatever of the aortic process, which is thus a slightly 

 later development of the sclerotome than is the notochordal process. 



From the ninth segment to the end, the nephrogenic tissue, the intermediate cell mass, 

 is connected to the lateral angles of the somites. In some instances it appears as if separa- 

 tion were just commencing and the w^all of the somite is broken at this point and the cavit y 

 is open. The attachment to the somite is on a line between dermatome and sclerotome 

 (text fig. 2). 



There is an intersegmental cleft between every two somites, but none cutting off the 

 nineteenth posteriorly. This nineteenth somite has a dennomyotome, a core, a sclerotome, 

 and a small cavity, and is almost the equal in size of the somite just ahead of it, but is still 

 fused posteriorly with the unsegmented mesoderm. In each intersegmental cleft is a 

 small amount of loose mesenchyme, which is also seen burrowing around the anterior and 

 posterior rounded ends of a few of the most anterior somites, working between them and 

 the ectoderm. In these somites mesenchyme is also burrowing in from the lateral edge 

 between the dermatome and the ectoderm. No mesenchyme is found between the medul- 

 lary canal and any somite (except the first, which is incomplete and fused with mesoderm 

 anteriorly), and each somite lies in contact medially with the wall of the medullary canal. 

 Also, no mesenchyme is found, except as stated above, between the dermatomes and the 

 ectoderm. 



The dermatomes and myotomes are epithelial in nature. The cell nuclei are huge. 

 round, and vesicular, and in the dermatomes are crowded toward the wall of the cavity 

 of the somite, leaving only the cell bodies to form the peripheral portion of the layer. The 



