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. XX VII.). 

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

 refringent spherules (6), 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 mieromeres form a spherical 



I 



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



1. The unsegmented egg. 2. Segmentation completed; the " ehitinogenous 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, a, egg cell, at first containing yolk 

 masses ; fo, 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; </, their nuclei; rf, 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 ; 0, the central (mesodermal) mass ; 5, 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. XX VIII. ). 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 



