DEVELOPMENTAL STIMULI IN THE CESTODA. 14! 



Viewing then the life history of such a Cestode from this point 

 of view we have first the union of the germ plasms followed 

 by a resting stage of varying duration. A stimulus furnished by 

 the contact with the sea-water when the eggs are laid brings about 

 the changes resulting in the six-hooked embryo. This embryo 

 when it receives a certain stimulus (condition of nourishment or 

 otherwise) from the intermediate host goes as far along the 

 course of development as the mid-larval stage and stops again. 

 On reaching the stomach of the final host the last stimulus of the 

 series is furnished and the adult condition attained. 



In those Cestoda which have an intra-uterine development, i. e., 

 forms in which a six-hooked embryo develops in the uterus, 

 we find the primary developmental stimulus intimately associ- 

 ated with the fusion of the germ plasms as in the fertilization of 

 most Metazoa, though in Bothriocephalus rugosns (Schauins- 

 land, '86) the intra-uterine development may not begin for sev- 

 eral weeks after the eggs have begun to pass into the uterus cavity. 

 In such cases the comparison between the primary " develop- 

 mental stimulus" and the developmental stimuli which follow is 

 of course not so patent, although it is none the less legitimate. 

 Whether what I have called a "developmental stimulus" in 

 these several cases shall be found eventually to be some new 

 condition which the egg or embryo meets or be found to be the 

 removal of some existing condition which has been inhibiting the 

 development is of no consequence here since the removal of an 

 inhibition may be spoken of as a stimulus, and since the impor- 

 tant thing is not the nature of the stimulus but the similar re- 

 action of the animal in each case. 



In conclusion I may say that no facts not already familiar to 

 students of Cestode life history have been set forth in the section 

 of this paper just concluded, nor can it be claimed that the ap- 

 parently two-fold nature of fertilization has not been in recent 

 years more than once promulgated. The comparison between 

 the reaction of the oosperm to the primary developmental stim- 

 ulus and the reaction of the larval stages each to its special 

 stimulus has interested me and it has seemed to me worth while 

 to attempt the formulation of Cestode life history from this point 

 of view. I also believe that the two-fold nature of fertilization 



