organs have not been carried beyond quite an early stage, but 

 so far as they go, they clearly agree with the description 



given by Van Beneden and Julin (33) for the buds of Peropho - 

 ra Listeri . 



Shortly after the peribranchial cavity has been 

 completely divided off from the branchial sac, a small col- 

 lection of cells appears between the two arms of the U-shaped 

 digestive tract and at the level of the duct of the "organe 

 refringent", almost at the point where the latter is connect- 

 ed with the stomach. The sexual organs arise from this lit- 

 tle sp?ierical mass of cells, which are at first but loosely 

 held together and identical in appearance with the mesodermal 

 cells of the blood. ?ig. 32, a, ./hich is precisely similar 

 to Van Beneden and Julin 's Pig. 5, b^, PI. XVI, shows the in- 

 timate relation between the cells of the blood and those of 

 the rudiment, so that there can be no doubt that the latter 

 are of mesodermal origin. The connection, already described 

 by the Belgian authors, of some of the peripheral cells of 

 the mass with surrounding mesodermal cells by protoplasmic 

 processes, is distinctly seen in the figure. A small ir;reg- 

 ular cavity is also shown in the centre of the clump of cells'- 



Some of the free mesodermal cells lying above the rudiment, 

 g . c . Pig. 32, a, are seen to be spindle-shaped; these soon 

 become joined, end to end, to form a solid cord united at 

 one end to the spherical mass of cells, and taking a course 

 parallel to that of the intestine, g.c . 32, b. This figure 

 represents a later stage, in which the cavity is considerably 



-54- 



