BY PROFESSOR W. A. HASWELL. 509 



As the embryo increases in size, it becomes somewhat oval, and 

 soon, when the long diameter is about 0-08 mm., the future 

 posterior end becomes marked by a slight depression, the anterior 

 end remaining rounded (Hg. 17). In this position a narrow cleft 

 appears running forwards towards the centre of the embryo. 

 This cleft does not open on the exterior, but is covered over by 

 the thin investing layer of cells. On either side of it is a mass 

 of cells from which the tail buds out; in front it terminates in a 

 slight transverse enlargement which partly separates off a dense 

 central mass of cells from two lateral masses. The enlargement 

 subsequently becomes the bladder of the excretory sj^stem. In 

 the lateral masses the rami of the intestine are developed at a 

 later stage. 



The central mass of cells is distinguishable (figs. 18 and 19) 

 into an anterior and a posterior portion which are not sharply 

 marked off from one another. The posterior portion is the rudi- 

 ment of the reproductive system. Of the anterior portion the 

 more ventrally placed cells form the rudiment of the pharynx, 

 the more dorsally placed that of the brain. Of these two organs 

 the pharynx appears first as a cylindrical cord of cells soon 

 distinguishable (fig. 20) into an investing layer and a central 

 core. The cells of the investing layer are the myoblasts of the 

 wall of the organ. They become enlarged, and increase in 

 number, though never becoming very numerous. In their plasma 

 are developed the radial muscular fibres of the pharynx. The 

 enclosed row of cells do not persist. They at first, fill the entire 

 lumen, but spaces appear, and the cells gradually become absorbed 

 and eventually entirely disappear (fig. 22). Whether they give 

 rise to the thin cuticle lining the interior of the pharynx is 

 uncertain. It is difficult to understand by what other agency 

 that membrane could be formed. 



At the anterior end, towards the ventral surface, when the 

 rudiment of the pharynx first becomes formed, a rounded group 

 of cells in all respects similar to the rest becomes enclosed by a 

 thin membrane formed of flattened cells. This is the rudiment 

 of the anterior sucker. 

 34 



