PLATE XLI. 



Fig. 1. (P. oblongus) irom above. Very early stage of ring, [pt = periblast]. The ring has just 



(96) become well defined everywhere and the embryonic fold is seen somewhat bulging centripetally. 

 The ring is nearly even in width everywhere, except for being a little wider as it enters the 

 embryonic region. 



Width of ring = .02 mm.; in embryonic region = .04 mm. 

 Cap almost perfectly circular. Diam. of cap = .55 mm. 



The thick wall (wreath) of periblast is not yet fully covered so that about one row of nuclei 

 is seen all around. The number of nuclei could not be accurately determined, so they were 

 put in at random guided by what was known of other cases. The outlines of only superficial 

 cells (epiblast) are seen, but nuclei of deeper cells are given. The epiblast is seen in profile along 

 the margin. The posterior half of the cap is thicker than the anterior, and this is shown by 

 heavier dots and lines. The cap should appear convex — but the lithographer could [not] be 

 instructed to shade it properly. 



The posterior [lower half of figure] half is the embryonic area, the anterior [upper] the pre- 

 embryonic area. 



The ring is now only .02 mm. [in width], later it becomes .12-. 15 mm. in width. 

 Fig. 2. [Section of] cap before appearance of ring, cleavage cavity shallow. 



(97) 

 Fig. 3. [Section] just before rmg. [6c = blastocoele; pb = periblast]. Embryonic area thicker 



(98) than pre-embryonic area. (Large yolk cells supplied). 



Fig. 4, [Section, be = blastocoele; pb = periblast]. Blastoderm thinned out much in anterior 



(99) half, where for some distance the lower layer is only one cell deep. Towards the anterior edge 

 the under layer thickens. 



