1885.] -09 [Packard. 



membrane during the early stage in its formation ; 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 characteristic appearance in Crustacea, the scorpion, myriopods 

 and hexapods. The membrane in question appears to have its homologue, 

 however, in the embryonic membrane of Apus, which we have described 

 in a foot-note on p. 161, of our first memoir. It 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 

 embryo. 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 mesoblast, which 

 are destined to form the muscles of the limbs and the ventral aspects of 

 the body, are represented in fig. 3. 



The mesoderm, as seen in fig. 3, is now differentiated into three sub- 

 laj'crs : 1, the somatopleure ; 2, the splanchnopleure, and 3, a sublayer 

 from which probably arises, in part at least, the connective tissue so re- 

 markably developed in the head of Limulus ; in its thickest portion at this 

 stage this innermost layer consists of about eight series of cells, which are 

 more loosely arranged than in the sublayers next to the epiblast. 



The yolk granules are minute, the largest granules not more than twice 

 as large as the nuclei of the mesoderm. The hypoblast cells are by far the 

 largest cells in the embryo, and at once attract attention by reason of their 

 size, and their deep color when stained ; the nucleus and nucleolus are 

 well marked. At this stage no hypoblast cells could be detected in the 

 yolk, nor any protoplasmic network connecting them. Those present 

 formed a dorsal row ranged next to the thin epiblast over about one- 

 quarter of the periphery of the ovum. In an earlier stage, however, the 

 yolk granules are contained in distinct polygonal cells, forming a network 

 extending through the yolk. 



The abdomen has not yet undergone segmentation ■, the incipient steps 

 are represented in fig. 2, where there appear to be arising five mesoblastic 

 segments (1, 2, 3, 4, 5). Between the first and second mesoblastic mass 



