THE DEVELOPMENT OP THE EARTH-WORM. 221 



cavity (figs. S, 4, 6, 7, ew). It is certain^ then, that before 

 the hollowing of the embryo by an invagination, which 

 produces the digestive cavity and the mouthy the layer 

 which is to become the endoderm is already easily recognis- 

 able. At that time, however, the aspect of all the cells is 

 still uniform, but when the invagination begins, a peculiar 

 change occurs in the endoderm cells. They increase much in 

 length, and become prismatic ; their nuclei approach the 

 extremities and project freely into the digestive cavity ; the 

 protoplasm becomes soft and filled with numerous albu- 

 minous corpuscles, a sure sign of the active nutritive changes 

 going on in it. In this stage the endoderm cells, which 

 never bear vibratile cillia, do not cover the digestive cavity 

 alone, but also line the buccal canal as far as its external 

 opening (figs. 8 and 10). 



Mention has already been made of two cells of the peri- 

 pheral layer, which become pushed into the interior, and 

 then covered by the flat cells of the ectoderm. This happens 

 near the aboral pole on the side which afterwards becomes 

 dorsal. They are very easily recognised when their external 

 surfaces still project freely on the surface by their size and 

 by their rather more dense protoplasm, and in the figs. 3 

 and 4 (cm) the way in which they become gradually 

 covered with flattened cells, which extend from all sides 

 towards a point of union, is seen. In figs. 6, T, 8, 9, en, 

 they are completely covered and have moved further in- 

 wards. Their longitudinal section is vvedge-shaped, with 

 the thin end diverted towards the periphery, and the base 

 bordering upon the layer of endoderm. They each contain 

 a large spherical nucleus. 



At the sides of each of these cells, between them and the 

 ectoderm, appear very soon two or three small, very thin, 

 disc-shaped cells placed one upon the other, with their bases 

 firmly adherent (figs. 3, 4, mes.) These cells, increasing 

 rapidly in number, group themselves in two rows or cords, 

 which, starting from the mesoblasts, are directed immediately 

 towards the opposite edges of the lentiform body, where 

 they turn up to join the oral extremity (figs. 5, 8, 9, 10, 

 11a, lib, mes). They thus together make a nearly complete 

 circle, interupted only behind by the two interpolated meso- 

 blast cells, and in front by the mouth ; they do not remain 

 long in this state, but first widen and then become thicker, 

 being now composed of two, three, or more rows of cells, 

 placed side by side, and of as many layers placed one upon 

 the other (figs. 11a, 12, 13). These cellular arches are the 

 rudiments of the mesoderm. 



VOL. XIX. NEW SER. P 



