68 ILLINOIS BIOLOGICAL MONOGRAPHS [262 



a whole within the shell. It was clear, however, that the development of the 

 cilia was parallel with the increase in the rapidity of motion, altho the muscular 

 system also had something to do with this motion The intestinal tract was 

 beginning to have a more definite boundary and to extend both forward 

 and back, and to a slight extent around toward the ventral side. During the 

 latter part of the seventh day the eyes began to be distinguishable as very 

 irregular brownish-red masses. They were almost crescent-shaped with the 

 center somewhat pointed and jagged. They lacked, however, the extreme 

 roughness of the adult form. 



By this time, also, the several folds of the integument were lost and the 

 worm had straightened out, doubled only once upon itself with one long fold 

 cutting thru the middle of the egg, so that the anterior and posterior ends 

 lay against each other at one side with the dark mass of the digestive tract 

 around on the other side. Thus, the whole ventral surface was turned toward 

 the center. 



During the eighth day one side of the egg showed a fine, dark line running 

 around a portion about the outer surface of the egg. This Hne became more 

 and more distinct and appeared like a crack in the shell. Its significance 

 was explained later. The rhabdites at this time were still very small and diffi- 

 cult to distinguish on account of their transparency, but were fast taking 

 the shape of the adult condition. They were arranged a few in each cell, but 

 the number for a cell was smaller than in the adult. The mouth and pharynx 

 rosette were also nearly complete. The two heavy, perfectly circular muscle 

 strands and the intervening rosette muscles were complete. The reproductive 

 organs were not developed, the atrial pore not broken thru, the papilla showing 

 not at all. 



The embryo was, by this time, very often rotating within the shell and was 

 almost constantly contracting one portion or another. The intestine now 

 extended well up toward the head and back almost to the end of the body 

 where it was broader and thicker. In a lateral view it could be seen extending 

 ventrally to the rosette and the ventral wall. Several rather small oil globules 

 appeared in the wall, giving the characteristic orange color. These, however, 

 were very few in comparison with the number which showed even immediately 

 after birth. The embryo had, by this time, evidently begun its metabolic 

 processes and was an almost perfect individual. 



During the first few hours of the ninth day the ring in the shell became 

 very conspicuous as a point of cleavage and gradually, bit by bit, the cap-like 

 portion pulled away from the rest, lessening the pressure within the shell and 

 causing a cessation in the movement for the time being. Later, the embryo 

 began struggling again, the force finally splitting the cap more than half way 

 around and allowing it to spring up so as to permit the little worm to squeeze 

 out thru the opening, leaving the empty shell intact. The cap fell back almost 

 into place The newly hatched worm very closely resembled the adult except 

 in size. It moved about very slowly at first, then more rapidly, and in a short 



