Royal Microscopical Society. 251 



forming eventually the head of the so-called Cysticercus pisiformis. 

 At' the anterior end the epidermis is folded inwards, and at this 

 point small calcareous particles appear, which become more 

 abundant ; later on, further changes of structure become visible in 

 the little parasite, as vessels circulating a clear fluid by means of 

 cilia beneath the subdermal muscular layers. Those destined not to 

 run to the advanced stage die down, the cyst becomes softened, 

 granular little masses of a mixed chalky and cheesy character are 

 formed, and the remains of the embryo are generally lost by this 

 retrogradation. A little later, some of the embryos escape into the 

 peritoneal cavity, being free of the cyst, and are termed " wandering 

 larvae," as yet imperfect in their development, which requires to be 

 perfected in another encysted stage. At the end of about the 

 eighth week the receptacle in which the head part is formed, and 

 where the crown of hooks and sucker-pits are situated, is capable of 

 retraction within the receptaculum, forming with the caudal sac the 

 complete development of this stage, and here they degenerate if 

 untransferred to their requisite host. It appears that these 

 imperfect or wandering larvae may be administered without 

 producing in the intestine the final phase of the Tsenia serrata, 

 though the administration of the secondly encysted form will 

 produce the Cestoid, the caudal vesicle being quickly destroyed by 

 the digestive function in the dog; the young entozoon rapidly 

 acquires length and segmentation, so that at the end of a fortnight 

 it has become more than 4 inches in length, and Dr. Cobbold puts 

 the period for the sexes to be matured in the segments at the 

 twentieth day or earlier. Dr. Cobbold points out that in the 

 Cysticercus fasciolaris found in the liver of rats and mice, the resting 

 larvae, or scolex, whilst in the mouse, frequently assumes the ttenioid 

 condition ; but in all such cases the incomplete development of the 

 sexual organs shows that the parasite is still a larva, and has not 

 yet gained access to its proper ultimate host. Dr. Aitken also 

 informs me that this corresponds with his experience. In most 

 cysticerci there is usually no trace whatever of the future repro- 

 ductive apparatus of the tapeworm. 



In reference to Botriocephalus, an entozoon found in man, but 

 the normal geographical distribution of which appears somewhat 

 limited, Dr. Cobbold, after stating Von Siebold's views, " that it is 

 not until the worm reaches the intestine of the ultimate host that 

 its segments acquire sexual completeness," says, and " this is a law, 

 as I have before had occasion to remark, which pervades all classes 

 rf parasites." He also notices that the scolex form of entozoon in 

 the sun- fish may even take on the taenioid condition to the extent 

 of 2 feet or more without acquiring sexual organs. Dr. Cobbold is 

 sceptical of the self-impregnation in the proglottides of Tsenia 

 echinococcus, though Leuckart supports such with his authority. 



