No. 3.] THE TAIL IN LUMBRICULUS. 327 



2. Mesoderm. 



I. The small cells that I have described as arising apart from 

 the neoblasts, and as present at a very early stage, are the first 

 of the mesodermic elements to become differentiated, and from 

 them are developed the circular muscles. They are to be found 

 — ventrally, laterally, and dorsally — close to the newly formed 

 ectoderm, but in greatest number in the ventral region, where 

 they are arranged with some degree of regularity in one or more 

 longitudinal rows on each side of the median line. The cells 

 are at first free in the body cavity, later the cell body becomes 

 flattened against the base line of the ectoderm and the nucleus 

 stands out into the body cavity. The next step in the develop- 

 ment is the appearance in the cell body of exceedingly faint 

 striations in a direction at right angles to the longitudinal axis 

 of the worm (Fig. 17). At first the protoplasm of the cell 

 extends only a little way on each side of the nucleus. In this 

 stage one sagittal section may show a number of nuclei and the 

 striated protoplasm of their cells, and the next, in correspond- 

 ing places, only a row of protoplasmic dots, which are the 

 pointed ends of the cells. The striations become more distinct 

 and are seen to be the lines of separation of the protoplasm 

 into fibrillae. The cells grow in length around the inside of the 

 ectoderm, becoming many times as long as their width. The 

 structure of the muscle cell is shown in Figs. 18, 19. In order 

 to see the first stage of the formation of the fibrillae, it is neces- 

 sary to examine specimens of about one day's growth. 



In this way the circular muscles arise around the inner sur- 

 face of the ectoderm. This mesoderm seems to be the only 

 tissue of its kind that penetrates the dorsal region. Here it 

 forms a loose tissue surrounding two spaces that are the founda- 

 tion of the dorsal vessel (Fig. 1 5). These lie at first far apart, 

 but gradually move together to the median line and fuse. 



The origin of the circular muscles, in this case, is in sharp 

 contrast to that described by Bergh (2) for the corresponding 

 structures in the embryology of Lumbricus. At a later stage 

 the lateral cell rows to which Bergh ascribes their origin are 

 present in the ectoderm of Lumbriculus, as in Lumbricus, but 

 . at this time the circular muscles have long been formed (Figs. 

 13, 20). 



