•.'(I 



Exei'dorv ()i'i,miis in J'xlcllostoiim St(niii 



second segment, and therefore very near the middle of the body region 

 and about five segments back of the middle of the region of the nephro- 

 tomes. The mesoblast, which alone interests us, is divided on either side 

 into four parts, the myotome, my, the sclerotome, scl, the neplirotoine, 

 nt, and the lateral mesoblast, Im. 



The m3'otome comes into direct contact with the ectoderm, there being 

 no mesenchyme between them, and this is true for all the segments 

 except the first four or five. There is no myocoel, but the centre of the 

 myotome differs from the periphera in being free from nuclei. In the 

 anterior segments muscle fibres are forming. Nothing of this is seen 

 in Fig. 1, although in the section next in front, which passes nearer 

 the middle of the myotome, some of the cells show signs of differentiation, 

 and this is true also in four or five of the segments further back. 



Figs. 2-8. — Seven consecutive sections from the middle of one nepbrotome to the 

 middle of the next, nc, the nephroccel shown in Figs. 2 and 8. Fig. 5 represents the 

 place where the two nephrotomes come together. 



The sclerotome, which is formed of compact mesoblast, is still con- 

 nected with the myotome, and its segmental character is further shown 

 in running through the series, by the fact that at regular intervals the 

 tissue here becomes much less compact. In about the last twenty-seven 

 segments the sclerotomes have not yet been formed. 



The lateral mesoblast has the appearance of loose mesenchyme, and 

 extends outward from the nephrotome with which it is continuous. 

 (This is shown only on the right in the figure.) In this section there is no 

 indication of a splitting into somatic and splanchnic layers, although, 

 as will appear later, the process has begun farther forward. 



The nephrotome lies just below the outer end of the myotome, from 

 which, in this case, it is separated by a narrow, artificial space, caused 

 presumably by the action of reagents. It is sharply marked off from the 

 surrounding tissue by the compactness of its walls, and it contains a 

 lather large cavity, the nephrococl. nc. In running tlirough the series 

 the segmental character of the ncphrocoels is very striking, as may be 



