Fig. 205.—Transverse section of the head of Dibothriocephalus latus. 30/1.
Fig. 206.—Fairly mature proglottis of Dibothriocephalus latus. 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 sac, and below the shell gland and ovary are seen. 15/1. (From a stained preparation.)
The eggs, which are deposited in the intestine and evacuated with the fæces, hatch in water after a fortnight or more; the embryonal integument (embryophore) of the oncosphere is provided with cilia; after bursting open the lid of the egg the oncosphere in its embryophore (fig. 207) reaches the water and swims slowly about; often it slips out of its ciliated embryophore, sinks to the bottom and is capable of a creeping motion; sooner or later it dies in the water. The manner and means of its invasion of an intermediate host are still unknown; yet we are aware that the larval stage (plerocercoid, fig. 208), which resembles the scolex and may reach a length of 30 mm., lives in the intestine, in the intestinal wall, in the liver, spleen, genital glands and muscular system (fig. [209]) of various fresh-water fish, the pike (Esox lucius), the miller’s thumb (Lota vulgaris), the perch (Perea fluviatilis), Salmo umbla, Trutta vulgaris, Tr. lacustris, Thymallis vulgaris (grayling), Coregonus lavaretus, C. albula (in Europe) and Onchorhynchus perryi (in Japan). The transmission of the plerocercoids from these fish to the dog, cat and man (Braun, Parona, Grassi and Ferrara, Grassi and Rovelli, Ijima, Zschokke, Schroeder) leads to the development of the broad tapeworm, the growth of which is rapid. In my experiments on human beings the average number of proglottids formed per diem averaged thirty-one to thirty-two for five weeks, with a length of 8 to 9 cm. According to Parona the eggs appear twenty-four days after man has been infected. Zschokke found the average growth in the experimental infection of man between 5·2 and 8·2 cm. per diem, and the person experimented upon by Ijima evacuated a piece of a Dibothriocephalus latus, 22·5 cm. in length, only twenty-one days after the infection.
Fig. 207.—Dibothriocephalus latus: development of egg. 1, segmentation complete; some cells of the blastosphere have migrated through the yolk and have flattened to form c, the yolk envelope; others form a layer of flattened cells (e) forming the embryophore; the remaining cells (d) of the blastosphere form the hexacanth embryo. 2, embryophore (e) is becoming thicker. 3, the ciliated embryo has been pressed out of the shell; s′, the operculum; c, the yolk envelope remaining in the shell (s); y, the yolk consisting of separate cells. 4, a free-swimming larva much swollen by the water. (After Benham and Schauinsland.)