166 



lu an embryo approximately 3 days old, rudiments of three pairs 

 of appendages are present, and the triangular area between the optic 

 discs and abdominal plate is closed over. The disc continues to grow 

 in the way described, but chiefly as before, by the emigration or inclu- 

 sion of superficial cells. The cells in the outer central portions of the 

 disc are large and granular, and actively multiplying. Spores at this 

 juncture make their appearance iu various parts of the embryo (see 

 University Circular, No. 0:5). 



We now pass to the »Egg-nauplius« stage (embryo 1 week old), as 

 the intermediate stages are not specially important, as regards the eye. 

 The optic disc has attained to the dignity of a lobe. It is an oval, 

 dense mass of ectoderm, swollen or protruding slightly from the sur- 

 face, and it is united closely to the «brain« , or ganglia of the antennae 

 (Fig. 5). 



XS. 



or. 



SOG. 



O'G. 



Fig. 5. Transverse section of the egg-nauplius, cutting the optic lobes [O.L.). 

 near their middle points, and the anterior end of the brain [S.O.G.] . 



O.L. = optic lobe. S.O.G. Supra-oesophageal ganglion. O.O. = gangliogen. 

 The retinogen, the superficial layer of large cells soon becomes very distinct. 



The next stage to consider is when there are 7 pairs of rudimen- 

 tary appendages. The optic lobes (using this term now instead of optic 

 discs) have grown very large and prominent, superficial layer of large 

 granular cells is clearly defined from the mass of cells below\ This 

 differentiation has begun in Fig. 5. The outer layer is the retinogen, 

 since all parts of the eye external to the basement membrane, are de- 

 veloped from it. The cell mass below, that is, the remainder of the lobe, 

 is appropriately called the gangliogen, since it gives rise to the ganglia 

 of the eye stalk. 



When to pairs of appendages are present, the optic lobes are of 

 enormous size. The retinogen is a concavo-convex cellular disc, con- 

 sisting of elongated cells arranged radially with respect to a centre in 

 the gangliogen. It is no longer one cell thick, except at its edges. A 

 delicate, structureless membrane is now developed in the optic lobe. 

 This is the basement membrane, a topographical boundary of 

 great importance, separating the retinogen or eye proper, from the 



