Here they become enclosed in a granular deposit from the surrounding tissues, which becomes in its turn enclosed in a connective-tissue coat. Within lies the solid embryo, the hooks of which in many cases disappear or become impossible to make out. In other forms, _e.g._ _Cysticercus limacis_, they remain visible, and then mark the anterior pole of the worm (fig. 98, _c._). The central part of the body next becomes transformed into a material composed of clear non-nucleated vesicles. Accompanying these changes the embryo grows rapidly in size; a cuticle is deposited by its outer layer, in which also an external layer of circular muscular fibres and an internal layer of longitudinal fibres become differentiated, and internal to both there is formed a layer of granular cells. With the rapid growth of the body a central cavity is formed, which becomes filled with fluid, and the embryo assumes the form of a vesicle. At the same time a system of excretory vessels, sometimes opening by a posterior pore, becomes visible in the wall of the vesicle. The embryo has now reached a condition in which it is known as a cystic- or bladder-worm, and may be compared in almost every respect with the sporocyst of a Trematode (Huxley). [FIG. 97. CYSTICERCUS CELLULOSÆ. (From Gegenbaur, after von Siebold.) _a._ Caudal vesicle. _c._ Anterior part of body. _d._ head.] [FIG. 98. CYSTICERCUS WITH SMALL CAUDAL VESICLE. A. Head involuted. B. Head everted. _a._ Scolex. _b._ caudal vesicle. _c._ (in A) six embryonic hooks.]