No. I.] THE GERM-BANDS OF LUMBRTCUS. 187 



rather abruptly outwards, and run forwards, soon becoming 

 broad bands that pass between the entoblast and the remaining 

 six cell-rows (compare Figs, i, 3, 4, 5, and 6). Posteriorly 

 the mesoblastic bands consist of a single layer, which splits 

 farther forwards into two layers (somatic and splanchnic) the 

 cells of which dovetail together with great regularity (Fig. 7). 

 Still farther forward the cells arrange themselves in groups, so 

 as to give rise to metameric cavities separated by rudiments 

 of the dissepiments, as shown in Fig. 3. 



The mesoblastic bands give rise to all the muscles and vessels 

 of the body, as well as to the ciliated funnels and outer invest- 

 ment of the nephridia, but the origin of the setigerous glands 

 is not so easy to determine. They arise at the same time with 

 the nephridia, the outer series lying just outside the outer cell- 

 row (Fig. 6), while the inner series is situated close to the row 

 of nephridia. The setigerous gland is at first very similar to 

 the rudiment of a nephridium, consisting of a single (?) large 

 central cell (which ultimately gives origin to the setae) sur- 

 rounded by a mesoblastic investment. It is extremely difficult 

 to determine certainly how the central cells arise, but I believe 

 them to be of mesoblastic origin on account of their behavior 

 with reagents, and because, in transparent surface-views, the 

 outer series of setigerous glands can be traced backwards until 

 it fades away in the mesoblast tihatlies outside the outer cell-row. 



VI. Growth and Concrescence of the Germ-bands. 

 Origin of the Teloblasts. 



Cleavage of the ovum is unequal, and in its general features 

 is similar to that of L. communis {trapezoides) as described by 

 Kleinenberg. The gastrula is formed by embolic invagination, 

 and at the time of infolding is saucer-shaped, the blastopore 

 occupying the entire ventral aspect. Closure of the blastopore 

 proceeds from behind forwards, and the anterior part, after a 

 stomodseal ingrowth of ectoblast, persists as the mouth. The 

 mesoblasts are differentiated at a very early stage, and some 

 time before the invagination, are pushed into the cleavage-cavity, 

 where they lie side by side at the posterior end. Each gives rise 

 to a row of (mesoblastic) cells that extends forwards near the 

 lip of the wide blastopore, and forms the rudiment of a germ- 



