165 



In tlie next phase the embryo is triangular or V-shaped, the optic 

 discs being vmited to the abdominal plate (which corresponds to the 

 apex of the V) , by a band of cells on either side. At this stage, before 

 any of the appendages are fully budded, a thickening of the optic disc 

 occurs (Figs. 2 and 3) . It begins in the centre of the disc, and is due 

 (1) to the emigration or crowding of cells from the surface (effected 



Fig. 3. Transverse section through the optic discs and egg, from same series 

 as Fig. 2. The left disc [O.DA] is cut through the centre. 



by cell-division in planes at right angles to a tangent at the surface); 

 (2) to delamination or division of cells in planes parallel with a 

 surface tangent. The initial thickening is due almost wholly to emi- 

 gration, but the second method soon becomes common. 



T.Cl. 



O.T). 



Y.C. 



Fig. 4. Transverse section through the optic discs and egg. (A few hours 

 younger than the egg-nauplius. Fig. 5.) Plane of section slightly oblique with respect 

 to long axis of embryo. The posterior portion of one [O.D.) is divided, the anterior 

 portion of the other. 



CM. = central area of optic disc, where cells are rapidly multiplying. 



T. Cd. = transverse cord, uniting discs. 



Y. C. = yolk cells (inner layer). 



Thus two solid ectodermal plates are formed, which are soon united 

 by a transverse bridge of cells (Fig. 4 T.Cd.) Elements from the 

 yolk pass forward to the optic disc, and it is almost certain that they 

 unite with it, so that to the processes just mentioned, by which the 

 disc is increased, must be added a third; namely the accession of 

 cells from the yolk (Fig. 4 Y.C.). 



