174 ZOOLOGY FOR MEDICAL STUDENTS chap. 



Microgametes received in the vagina unite with the macrogametes 

 coming from the ovary by way of the oviduct and the resulting zygotes, 

 with yolk-cells from the yolk-duct, become encased in shells by the 

 activity of the shell-gland and then pass forwards into the uterus in 

 which they accumulate. The accumulation of eggs brings about changes 

 in the appearance of the uterus which are very characteristic. Its 

 side walls become gradually distended to form pockets which gradually 

 increase in length, becoming irregularly branched as they do so, until 

 they almost reach the lateral boundary of the proglottis. Eventually 

 the greater part of the whole proglottis is occupied by the branches of 

 the uterus (Fig. 80, A and B), the other reproductive organs shrivelling 

 up and becoming quite inconspicuous. • 



This condition, in which the proglottis is little more than a packet 

 of zygotes or fertilized eggs, is found in the hindmost proglottides in the 

 chain, i.e. the oldest proglottides, for the proglottides originate, just as 

 do the sexual individuals of Myrianida, by being cut off from the hinder 

 end of the asexual portion or scolex. Eventually the old proglottis, 

 full of eggs, is dropped off. It passes to the exterior with the faeces of 

 the host : it may show signs of life for some time but before long it dies 

 and disintegrates and the eggs are scattered abroad, nothing more hap- 

 pening unless the egg is swallowed by the appropriate host animal. 



Cestode Life-Histories 



Taenia serrata, one of the commonest tape-worms of the dog, affords 

 an excellent example of the typical cestode life-history. The further 

 development of the egg takes place if, and only if, it be swallowed by a 

 rabbit. In this event there hatches out within the alimentary canal 

 a small rounded larva, provided with six sharp blades by means of 

 which it cuts its way into the wall of the alimentary canal. Eventually 

 it reaches the blood and is carried off in the blood-stream towards the 

 liver. It apparently usually leaves the blood-vessel within that organ 

 and then may migrate for some distance before it finds a suitable spot, 

 such as the lining of the body-cavity, in which to settle down. 



It now grows into the cysticercus or bladder-worm — a semi-transparent 

 lemon-shaped vesicle about the size of a pea and full of clear fluid. 

 Through the wall of the vesicle can be seen shimmering an elongated 

 whitish body which projects inwards from one pole. In a well-infected 

 rabbit many of these bladder-worms may be seen scattered about in 

 the lining of the body-cavity. If a fresh bladder-worm be slightly squeezed 

 between the fingers the whitish structure within it shoots out, and it 



