No. 3.] STUDY OF STENOSTOMA LEUCOPS O. SCHM. 295 



chain before the pharynx of the younger bud opens to the out- 

 side. 



The first sign of the bud is a very shght circular constriction 

 of the integument which marks the anterior end of the bud. Just 

 as this appears, a sHght swelHng is noticed in the Hving form on 

 the medial side of each of the lateral nerves. These swellings 

 gradually increase in size as globular masses of cells (Fig. 23). 

 They grow toward the median line dorsal to the intestine. 

 When they have nearly come together each puts out a mass of 

 fibres which unite in the median line, and thus form the com- 

 missure which joins the lobes of the brain. 



When the ganglia which are to form the new brain are about 

 20 /x in diameter, the integument of the side of the body just 

 anterior to them may be seen to be pushing in to form the new 

 ciliated pits. These pits gradually grow deeper and wider and 

 embed themselves in the mass of ganglion cells. When the 

 ciliated pits are well developed the dish-shaped organs appear. 

 The nerves connecting these organs with the brain are then 

 developed and can be seen in the living worm. 



At the same time that the ganglia begin to form, a small mass 

 of modified parenchyme cells may be seen between the intestine 

 and the ventral body wall, and directly opposite the newly 

 formed ganglia. This mass grows toward the middle of the 

 intestinal cavity, pushing the wall of the intestine before it 

 (Fig. 23). 



A spherical lumen next appears in this cell mass, and as the 

 mass gradually lengthens until it becomes ovoid in form, the lu- 

 men becomes long and narrow. By this time the outer border 

 of this cell mass has become definite, and the periphery of the 

 new pharynx is thus plainly marked. The cells around the 

 lumen arrange themselves radially (Fig. 24). 



The new pharynx is now so situated that the anterior end is 

 next to the ventral body wall, and the long axis stands at an 

 angle of about 30° with the long axis of the body. When the 

 lumen of the pharynx first appears, a slight indentation in the 

 ventral body wall may be noticed a short distance in front of 

 the anterior end of the new pharynx, and just posterior to the 

 furrow which separates the bud from the mother. This inden- 

 tation deepens until it reaches the new pharynx, when the 

 muscular wall of the integument is ruptured, and the borders 



