402 Wilson: [Vol. vi. 



the last, as shown in the figures. Later bilateral divisions of 

 c^, d^ are shown in Figs. 45 and 59, but I have not followed 

 them in detail beyond the seventh division. There can be no 

 doubt that the cross gives rise in large part to the cerebral 

 ganglion. For a further account of it see p. 421. 



The descendants of the intermediate girdle-cells have mean- 

 while continued to divide so as to fill the interspaces of the 

 cross, but I have thus far not attempted to follow this history 

 in detail. 



2. History of the Head-Kidney. 



We may now return to the history of the cephalic nephro- 

 blasts {c^-^, d^-^). From their first formation these cells are 

 overlapped by the adjoining cells, and as the development pro- 

 ceeds they soon sink below the surface, and are finally entirely 

 covered in (Figs. 38 to 45). The protoplasm meanwhile be- 

 comes clear and vacuolated and shows so marked a contrast to 

 that of the adjoining cells that after a little practice the nephro- 

 blasts may, in good preparations, be distinguished at the first 

 glance. In the Schneider-acetic preparations every stage of the 

 enclosure may be clearly followed, especially in the side-views, 

 where the exact relation of the nephroblast to the adjoining 

 cells may be seen in optical sections by rolling the embryo 

 from side to side. The nephroblasts meanwhile become more 

 elongated and somewhat pointed at the ends. After sinking 

 below the surface they slowly migrate outwards and downwards 

 towards the prototroch, forcing their way between the outer 

 layer of cells and the four entomeres. In this way they pass 

 downwards until they lie quite in the lower hemisphere below 

 the prototroch (Figs. 74-76), where the remainder of their 

 development is accomplished. 



After taking up its position below the prototroch (Fig. 75) 

 the nephroblast rapidly elongates, extending itself forwards and 

 backwards between the outer cells and the entomeres. It is 

 thus converted into an elongated organ which extends about 

 half-way around the body (Figs. 62, 6$, 75-77, 79, 80-84). 



Mention has already been made of the vacuolation of the 

 protoplasm of the nephroblasts. As far as I have observed, 

 the vacuoles always appear at one side of the cell (Figs. 74, 75), 

 the nucleus being crowded to one side. As the nephroblast 



