580 SYDNEY J. HICKSON. 
in the endoderm of the canals, or (b) in the degenerated cell- 
mass of the trophodise of an escaped embryo (Plate XX XVIII, 
figs. 2 and 38). 
2. As an ovum growimg in one of the ordinary canals 
enlarges it pushes out the endoderm and ectoderm of the 
canal in which it is formed, and thus forms for itself a diverti- 
culum connected with the canal by a narrow aperture. At 
the same time the endoderm of the canal wall in the immediate 
neighbourhood of the aperture of the diverticulum becomes 
thickened and throws out five radial pouches, which embrace 
the proximal pole of the diverticulum containing the ovum 
(Pl. XXX VIII, fig. 5). 
3. The five radial pouches thus formed throw out secondary 
pouches and become flattened against the proximal pole of the 
ovum, and thus form a nourishing lenticular mass of cells— 
the trophodisc! (Pl. XX XVIII, fig. 7). The protoplasm of the 
ovum is at first clear, but with the formation of the trophodisc 
it becomes filled with small yolk-spheres. It possesses no 
trace of a vitelline membrane, but is surrounded and protected 
by a chorion, the attenuated ectoderm and endoderm of the 
primitive diverticulum. The germinal vesicle is large, spherical, 
or oval in shape, limited by a well-marked membrane, and 
contains a large germinal spot, and a sparse chromatin mesh- 
work with nodal thickenings (Pl. XX XVIII, fig. 6). 
4. When the trophodisc is fully formed and the lumina of 
its constituent pouches obliterated, the germinal vesicle be- 
comes oval or irregular in shape and travels towards the distal 
pole of the ovum. At this stage all the yolk-globules are 
spherical in shape, the larger ones are found only in the 
proximal and peripheral regions of the egg, those found in the 
immediate neighbourhood of the germinal vesicle being smaller 
and very numerous (Pl. XXX VIII, fig. 7). 
5. The germinal vesicle travels until it reaches the chorion 
covering the distal pole of the ovum, when two polar bodies are 
protruded. The germinal vesicle then flattens itself against 
the chorion, and apparently remains in that position for some 
‘ This name was kindly suggested to me by Professor Lankester. 
