64 ILLINOIS BIOLOGICAL MONOGRAPHS [258 



that the pressure is great and only a small amount of displacement will cause it 

 to be extruded. The canal leading from the atrium to the external pore is short 

 and held firmly in place by the surrounding tissues, but possesses a most 

 elastic wall so that very quickly, when the pressure is sufficient, it enlarges many 

 times to allow the passage of the egg. The pore itself has a very flexible wall, 

 the epidermal cells flattening and lengthening to an almost unbelievable de- 

 gree. At the close of the sexual period the organs are very much depleted 

 and have shrunken, loosing also the definiteness and toughness of the walls 

 so that the whole appearance is very different from that of the earlier period. 



DEVELOPMENT 



Individuals carrying eggs were first noticed about the middle of November 

 and a few were found in every haul made during the next four weeks. One 

 more appeared on the twelfth of January, but none later. In all cases these 

 individuals were not as generaUy opaque as those not sexually mature, i.e., 

 the intestine was not so clogged with food, and oil globules were present in 

 only small numbers. The body was also much more slender, indicating either 

 that the regular amount of food had not been taken or else that the strength 

 was being used for the nourisliment of the egg. Altho sensitive, they moved 

 around less actively than the others and seemed almost to be lacking in vigor. 



The egg, which develops singly, is carried in the atrium seminaHs and is 

 visible because of its opaqueness even to the naked eye. It appears as a thick 

 whitish spot, a tiny spherical knob causing the surface of the body to bulge 

 a little on both dorsal and ventral surfaces. Under low magnification it 

 shows as a red-brown sphere, surrounded by a capsule made by the wall of 

 the atrium. A little smaller than the pharynx rosette, this capsule generally 

 lies just posterior to it and near the middle line of the body On account of 

 the transparency of the body and the heaviness of the egg, it appears as con- 

 spicuous as the rosette itself. The egg capsule though retaining its relative 

 position in the body parenchyma is very movable, recalling a baUoon buffeted 

 about in all directions but anchored to one spot by its tether, which latter is 

 comparable to the short canal leading from the atrium down to the pore on 

 the ventral side of the body. This pore is generally almost invisible, but, under 

 some conditions, it is drawn up into a rather large papiUa, just posterior to 

 the pharynx. This drawing up is due to the contraction of muscles in its walls 

 and in the surrounding integument. 



As soon as they were discovered, animals carrying eggs were isolated in 

 small watch glasses, where they were kept in about five cc. of water taken from 

 the aquarium jar in which they were found. These jars had been filled with 

 water containing a greater or less amount of algae, water plants, many Crusta- 

 cea, other rhabdocoels and an inch or more of mud and sticks at the bottom, 

 in every way very much like the natural pond except in temperature, which 

 was slightly variable being the room temperature. The temperature was, of 

 course, very different from that of the water under the ice in the ponds, but 



