242 DUMPUS. [Vol. V. 



lobes. The entodermal mass has undergone considerable in- 

 crease in size, and the contained yolk is manifestly different 

 from that without. It is as if the corners had been digested off, 

 the granules appearing quite spherical. The entoderm cells, or 

 nuclei — for limiting cell walls are extremely difficult to resolve 

 — are often in pairs, as if undergoing division. Immediately 

 under Gm. (PI. XVII, Fig. 5) are seen larger mesodermal cells. 

 From these other cells are seen to extend anteriorly, and, to a 

 slight extent, posteriorly. Chromatin grains are associated with 

 the mesodermal elements. 



In the transverse section through the closed blastopore (Fig. 6), 

 which is somewhat more highly magnified, the arrangement of 

 the germinal layers is clearly seen. The ectodermal cells (Ect.) 

 form a layer one or two cells in thickness, which, at the base 

 of the blastoporic scar (Gm.), give place to the larger, turgid 

 mother-cells (?) of the mesoderm (Mes.). Extending laterally 

 from these, the mesoderm forms a somewhat broken tissue, 

 lying immediately under the ectoderm. The entodermal cells 

 (Ent.) are generally slightly different from those of the other 

 layers, though they cannot always be separated with absolute 

 certainty. 



A cross-section drawn through the egg (Fig. 4) shows the 

 evenly disposed ectodermal cells, which are beginning to thicken 

 at the ganglia (C. 4 ). The median groove is clearly presented at 

 Mg, while the mesoderm forms a single, unbroken layer on 

 either side the median line. Though the chromatin elements 

 are much less abundant here than farther forward, they are 

 irregularly aggregated in the median line and in two lateral 

 masses. 



A section through the stomodaeum (Fig. 3) shows the eleva- 

 tions of ectoderm at the ganglionic centres as well as at the 

 insertion of the appendages. The mesoderm is still an evenly 

 disposed band, while the chromatin grains are almost absent. 

 At Fig. 2, however, the chromatin is again abundantly found, 

 while but one or two mesodermal nuclei are present. At Seg.' 2 

 is a thickening of the ectoderm, which I consider to be the sec- 

 tion of the intercalated, crescentic appendage. 



Before leaving this stage, a comparison with Astacus may 

 prove valuable. The cray-fish now has a lumen in the centre 

 of the entodermal mass, the cells having actually devoured the 



