DEVELOPMENTAL STIMULI IN THE CESTODA. 131 



the mass. The yolk-producing organ consists of follicles densely 

 packed with yolk cells and distributed in the proglottid as the 

 figure indicates (yg, Fig. i). 



Fig. i represents a specimen killed under pressure and in 

 which the uterus had been ruptured and the eggs squeezed from 

 the oval hole represented by the dark outline in the center of the 

 proglottid. Very much the same sort of a hole is left when the 

 proglottid ruptures itself at this point in the normal egg-laying. 

 The extent of the uterus cavity is indicated by the outline (it) in 

 the figure. The condition of the intact uterus and the place of 

 its rupture will be explained in describing the egg-laying of the 

 ripe proglottids. 



ACTIVITIES OF THE MOTILE PROGLOTTIDS. 



The proglottids of Crossobothrium laciniatum are an extreme 

 case of what is usually termed the "motile" condition. So 

 definite are their movements and activities that one is constantly 

 thinking of them as though they were individual animals of a 

 species entirely distinct from the parent scolex. When observed 

 in the chyle they are seen writhing about, contracting and elon- 

 gating rhythmically and bending their bodies into an arch along 

 the axis of breadth, now one way, now another. If we measure 

 the maturity of a proglottid by its size and the number of eggs 

 accumulated in the uterus the conclusion is reached that the pro- 

 glottids as taken from the spiral-valve are of diverse ages, for one 

 finds a considerable variation in the number of eggs accumulated 

 in the uterus and a correlated variation in the size of the pro- 

 glottids. 



When placed in clean sea-water the smaller proglottids do not 

 lay their eggs, while the large ripe ones will almost immediately 

 do so. These facts seem to indicate that the proglottids may be 

 shed off from the strobila some time before they are ripe and 

 remain in the shark's intestine until they are fully loaded with 

 eggs and ready for the laying. The enormous number of pro- 

 glottids usually found in a single spiral-valve is another fact in 

 favor of this conclusion. On the other hand fully mature pro- 

 glottids are frequently found on the end of a strobila (Fig. 5), 

 showing that they may mature while still attached to the parent 

 stock. 



