OF OECANTHUS AND TELEAS. 243 



completion of the nervous invagination the mesoderm again unites in the middle line, and 

 later becomes separated into its permanent somatic and splanchnic layers. 



The cavities of the cup-like pockets of mesoderm extending into the appendages are at 

 first in direct communication with the yolk cavity, but they soon become cut off from the 

 main cavity by cross partitions of mesoderm which close the opening by an annular con- 

 striction. (PI. 18, figs. 17, 20, 21, 24; pi. 22, figs. 23-25; pi. 23, figs. 11, 12, 13.) 

 The mesodermic lining of the antennae and upper lip is entirely similar to that 

 of the other appendages. In the case of the upper lip, however, the formation of the 

 ingrowth is somewhat different : the mesodermic sheet is pushed off from the ectoderm by 

 the ingrowing stomodaeum (pi. 23, fig. 13) and, as the latter extends inward taking a 

 longitudinal direction, the mesoderm grows around it from its dorsal side, the two meso- 

 dermic folds coalescing in the median ventral line ; from this portion of the mesodermic 

 layer the lining of the cavity of the upper lip is derived. In the head region the ecto- 

 derm is thrown into folds by the proliferation of the cells of certain tracts. These 

 thickened tracts give rise to the cellular mass of the supra-oesophageal ganglia. 



The fibrous portion of the brain as well as that of the ventral cord is undoubtedly fur- 

 nished by the cell walls of the ganglionic cells (pi. 22, fig. 2). In pi. 18, fig. 24, are shown 

 the internal ectodermic folds of one side of the head, o. being the lumen or cavity of the 

 head from which the mesodermic elements have receded ; fcl 1 . the lower fold lying against 

 the invagination sk. of pi. 18, fig. 15; fcF. the upper, outer fold which, to judge from its 

 size, probably gives rise to most of the brain mass. Contrary to the observations of 

 Hatschek (23, pp. 9, 10), I find the mesoderm extending into the head region, where it is 

 also provided with a body cavity (pi. 23, fig. 13,1). In pi. 22, figs. 18, 19, 20, 21, 22, are 

 shown sections through the end of the abdomen and the proctodaeum of an embryo of 

 about the stage outlined in pi. 18, fig. 21. The proctodaeum projected beyond the end 

 of the dorsal cj of the ^, and consequently the first few sections pass through the procto- 

 daeum alone. In figs. 18 and 19, pi. 22, the mesoderm is seen as a crescentic layer only 

 partly surrounding the thick ectodermal wall of the proctodaeum on its dorsal side, 

 whereas, a little farther from the end (fig. 20) it has nearly enclosed it. In fig. 21, the 

 region of the free end having been passed, the amnion is seen in section extending over 

 the anal stylets, while the mesoderm is confined to the ventral surface of the germinal 

 band. In the case of the proctodaeum, as was also seen in that of the stomodaeum, the 

 mesoderm grows around the invaginated mass from the dorsal side and its limbs coalesce 

 in the median ventral line. Sections through the maxillary region at this stage (pi. 22, 

 fig. 23) show the first maxilla to be a three-lobed, much broadened appendage. 



For convenience of description, 1 will first give a sketch of the development of the 

 embryo during the third stage as a preliminary to the more detailed account of the devel- 

 opment of the organs. The general relations of the embryo to its membranes and to the 

 yolk mass at the close of the second stage may be briefly stated as follows. The egg has 

 increased much in volume and the outer chorion has ruptured, exposing the inner hvyer. 

 The embryo extends along the concave side of the egg for two-thirds of its length, with 

 its abdominal end folded upon itself in a ^ fold, from the ventral arm of which the amnion 

 extends over the ventral surface of the embryo as an entire sheet, united with the embryo 

 along its lateral margins and the dorsal edges of the head fold (pi. 20, figs. 7 and 8). On 



