DEVELOPMENTAL STIMULI IN THE CESTODA. 139 



tuted almost word for word for a description of what occurs in 

 one of these Cestodes. 



In addition to the phenomenon of a resting stage in their early 

 development the Cestodes just mentioned, and indeed all other 

 Cestoda, present in their subsequent life history a feature which 

 I have found interesting when considered in connection with 

 the primary "developmental stimulus" which starts the oos- 

 perm upon its course. Such a consideration of the subsequent 

 facts of Cestode life history may perhaps widen our concep- 

 tions regarding the nature of one of the two phenomena 

 which exist side by side in normal fertilization. How this is 

 will be easily apparent if we recall the life history of Crosso- 

 bothriuin or any Cestode having a similar extra-uterine develop- 

 ment. 



The female reproductive organs of each proglottid produce 

 ova which on being fertilized become surrounded by their yolk 

 supply and encased in a tough shell. Without undergoing any 

 developmental changes they accumulate in the uterus where they 

 remain in this condition until the time of egg-laying. They are 

 thus of very diverse ages if we date the age of each from the 

 time of the entrance of the spermatozoon, but all are in the same 

 resting unicellular state. We have here the union of the germ 

 plasms, but the stimulus to development delayed for a period 

 which is long or short, depending upon the age of the individual 

 oosperm. The stimulus to development is normally found in the 

 contact with the outside sea-water when the eggs are shed, for 

 the cleavage begins only when they are thus set free. Develop- 

 ment proceeds so far as the six-hooked embryo stage when death 

 ensues unless the proper host is found. In the case of Crosso- 

 bothrium there is perhaps a primary intermediate host between 

 the six-hooked embryo and the squeteague in which case the 

 six-hooked embryo which infects this intermediate host receives 

 a stimulus to development which sends it so far as the resting 

 stage which is attained in that particular host, and here it stops 

 and eventually comes to naught unless it is carried into the next 

 host, the squeteague, where it finds a new stimulus to further 

 developmental changes and attains in the cystic duct of this fish 

 its development to the full structure of a tetrabothrian larva. 



