1885.] Embryology. 723 
a 
tion; after a while new matter is added to the interior which is 
structureless, so that the cellular appearance is only superficial. 
In my comparison of this membrane with the serous membrane, 
I certainly exaggerated its resemblance to the serosa of insects, as 
the latter is a much more delicate membrane, and with a charac- 
teristic appearance in Crustacea, the scorpion, myriopods and 
hexapods. The membrane in question appears to have its homo- 
logue, however, in the embryonic membrane of Apus, which we 
thus appears that this supposed point of resemblance in Limulus 
to the Tracheata is removed. 
A longitudinal section of the embryo of Limulus is represented 
by Fig. 2. The section passes through the blastodermic disc 
(ventral plate) and the indications of the appendages, on one side 
of the median line of the body. The epiblast entirely surrounds 
the yolk, forming a thin layer with nuclei, the cell walls not being 
distinct, while the nucleolus consists of a number of granules. 
The nuclei are two deep only on the cephalic portion. of the em- 
bryo. The blastodermic disc does not extend quite half way 
around the egg. The six pairs of appendages are well developed, 
increasing in size from the first to the last pair. The mesoblast 
is now well developed; the nuclei well marked, but the cellular 
walls more or less effaced. The mesoblastic arthromeres are now 
well indicated. The somatic cavities are well marked in each 
appendage ; the somatopleure is from one to three cells deep ; the 
splanchnopleure is formed usually of two layers of cells, and is 
more or less continuous at the ends of the somatic cavities with 
the somatopleure. The relations of these divisions of the meso- 
protoplasmic network connecting ose present formed 
a dorsal row ranged next to the thin epiblast over about one-quar- 
ter of the periphery of the ovum. earlier stage, however, 
