136 



THE CESTOIDEA 



but their interpretations of certain phenomena do not agree 

 with one another, nor with those of Schauinsland for Bothrio- 

 cephalus. The best general account is that of van Beneden for 

 T. serrata. The egg, when it passes into the uterus, consists of the 

 thin egg-shell, deposited in the ootype, surrounding a transparent 

 non-cellular yolk, in which is embedded the egg-cell (Fig. XXVII. ). 

 The first segmentation gives rise to two blastomeres, one filled with 

 refringent spherules (b), and the other faintly granular ; the latter 

 continues to segment, and ultimately three large cells (c) and numer- 

 ous smaller ones are produced. The large cells increase in size and 

 give rise to a "yolk envelope " enclosing the "granular cell " (b) and 

 what remains of the original yolk (Fig. XXVII. 2). The whole egg 

 has greatly increased in size. The micromeres form a spherical 



FIG. XXVII. The early development of T. serrata (after v. Beneden). 



1. The unsegmented egg. 2. Segmentation completed; the " chitinogenous layer" is 

 growing over the embryo. 3. The embryo exhibits an outer layer and central mass. 4. The 

 completed egg, with the six-hooked embryo within, c, egg cell, at first containing yolk 

 masses ; b, one of the two first blastomeres, loaded with refringent spherules (which are not fat) ; 

 c, the three macromeres, derived from the other blastomere, uniting to form the " albuminous 

 coat" or yolk envelope; c', their nuclei; d, embryonic mass, derived from the mieromeres ; 

 e, chitinogenous layer, derived from some of these, which overgrow the embryo, giving rise to 

 the striated coat in 4 ; /, outer layer (ectoderm) of the embryo, destined t^ give rise to the six 

 hooks ; gr, the central (mesodermal) mass ; s, shell, formed in the ootype ; y, yolk, formed by 

 vitellaria. 1, 3, 4 are equally magnified. 2 rather less. 



mass, and two cells at one end flatten out to form a cap ; these cells 

 divide further, and gradually enclose the remainder ; the superficial 

 layer of cells thus produced gives rise to a chitinoid coat ; the central 

 mass becomes the hexacanth embryo (Fig. XXVII. 4). This chitinoid 

 coat, the homologue of the ciliated mantle of Bothrioc. latus, takes on 

 a characteristic structure in each of the sub-families of the Taeniidae 

 (Fig. XXVIII.). In the Taeniinae it is striated ; in the Hymenolepinae 

 it is homogeneous and refringent (1, 2); while in the Anoploceplialinae 

 it undergoes remarkable changes, becoming drawn out into two horns 

 on one side, which may even cross, scissor- wise, forming the peculiar 

 " pyriform apparatus " referred to above (Fig. XXVIII. 3, 4). 



The development as far as this stage takes place in the uterus, 

 while the proglottid still forms part of the strobila ; and in the 

 Taeniidae, and probably in all the Tetracotylea, the eggs of different 



