DIBOTHRIOCEPHALUS LATUS 



215 



shortly before its junction with the oviduct; the vitellaria are 

 in pairs ; in shape they resemble the wings of a butterfly and they 

 lie ventrally in the medullary layer ; the shell glands lie in the 

 posterior division of the vitellaria ; the uterus, forming numerous 

 transversely-directed convolutions, passes ventrally from the vas 

 deferens towards the front. Eggs (fig. 141) large, with brownish 

 shells and small lids ; 0*068 0*071 : 0*045 mm., the germinal cells, 



FIG. 139. Transverse 

 section of the head of Di- 

 bothriocephalits latus. 30/1. 



FIG. 141. Egg of Dibo- 

 thriocephalus latus. 240/1. 



FIG. 140. Fairly mature proglottis of Dibothrio- 

 cephalus latus. 15/1. From a stained preparation. 

 The follicles of the vitellaria are at the sides ; 

 the uterus, filled with eggs, is in the middle, also 

 the vagina (the dark stripe passing almost straight 

 from the front to the back), and the vas deferens 

 (almost hidden by the uterus) above. In the centre 

 is the cirrus pouch, and below the shell gland and 

 ovary are seen. 



which are already, as a rule, in process of segmentation, are sur- 

 rounded by numerous large yolk cells ; the proglottides nearest 

 the posterior extremity are frequently eggless. 



The eggs, which are deposited in the intestine and evacuated 

 with the faeces, hatch in water (Schubart, Bertolus, Knoch) after 

 several weeks ; the embryonal integument of the oncosphere is 

 provided with cilia ; after bursting open the lid of the egg the^onco- 

 sphere reaches the water with its integument and swims slowly 

 about (figs. 142 and 143) ; often it slips out of its ciliated mem- 



