Procephalic Lobes of Epeira Cinerea. 441 



111 following the development of this fold we find that it arises, as 

 has been previously pointed out, as a thickening of the edge of the 

 plate which becomes turned upward and backward over the lobes. 

 Its continued growth causes it to advance toward the median line, and 

 in a posterior direction over the lobes, the thickened portion remain- 

 ing on the edge of the fold. The fold may consequently be described 

 as an ectodermal mantle consisting of two parts, an inner and an outer. 

 The outer portion of the mantle consists of a single sheet of cells 

 which extends dorsalwards and becomes continuous with the ecto- 

 derm of the dorsum. The inner part is several layers of cells thick 

 and lies close to the cerebral lobes, being continuous with the dorsal 

 lip of the anterior optic invagination. On account of the infolding 

 of the ectoderm to form the inner layer of this structure, the thick- 

 ened optic plates come to lie in an inverted position near the edges 

 of the fold, and form the retinas of the anterior median eyes. 



The pressure of the neuromeres which have crowded into the region 

 of the head and thorax has brought about a stiU greater concentration 

 of the neural elements. In the head the various ganglia are com- 

 pacted into a single mass of nerve tissue in which the individual 

 l)arts can be distinguished with difficulty. The anterior optic invagi- 

 nation has also increased in size, its walls having become so much 

 thicker that its lumen is completely obliterated and the entire struc- 

 ture has come to lie, to a greater extent, underneath the cerebral 

 lobes. 



Another marked feature of this stage is the lessening of the super- 

 ficial area of the hea<l. During the first stages in the growth of the 

 embryo, the cephalon lies as a broad plate on the surface of the yolk. 

 As its development progresses, and the fusion of originally distinct 

 parts occurs, accompanied by the sinking of certain of the superficial 

 portions below the surface, a reduction of the superficial area of the 

 head ensues. This process is particularly evident in the narrowing 

 of the anterior portion of the head. 



Stage IX. Fig. 30. — In surface views of this stage the optic fold 

 is seen to cover the anterior half of the o])tic lobes. Its posterior 

 margin forms an inverted \ , the arms of which diverge considerably. 

 The more anterior portions of the fold have met in the median line. 



