132 Scientific Proceedings^ Royal Dublin Society. 



into this chamber, the reflected ectoderm running up the oeso- 

 phageal wall, and driving the endoderm before it. The second 

 pair of mesenteries appear in the larger chamber as longitudinal 

 ridges (the supporting lamella), which cause no elevation of the 

 endoderm. 



The ectoderm reflected into the larger of the two primary 

 chambers is pushed down by the growth of the second pair of 

 mesenteries. From it are formed the filaments for these mesen- 

 teries, while the remainder of the original tract splits into three 

 divisions. 



The middle division (his chamber c), is not pushed entirely to 

 the edge of the oesophagus ; later in life, when the fourth pair of 

 mesenteries is well developed, this tract grows once more nearly to 

 the upper limit of the oesophagus. The lateral divisions a and h are 

 pushed to the edge, but after the sixth pair of mesenteries has 

 appeared they begin to grow up again. 



"When the mesenteries of the third pair are well advanced the 

 ectoderm is reflected into the smaller of the two primary chambers, 

 and runs up the oesophageal wall nearly to the top of the chamber. 

 The mesenteries, when they begin to grow down, carry a part of 

 the ectoderm along their free edges as very slender filaments. As 

 the fourth pair of mesenteries continues to increase in size, the 

 tract of ectoderm which belongs to the chamber c extends farther 

 upward. 



The fifth and sixth pairs of mesenteries appear simultaneously. 

 The filaments for the fifth pair are probably formed from the lateral 

 portions of the reflected ectoderm (termed by Wilson " lobe x ") 

 after it has been divided by the completion of the third pair of mes- 

 enterics. The filaments for the sixth pair, it seems, will be formed 

 from the tracts of ectoderm which belong to the chambers a and 6. 

 These tracts were in most larvae pushed completely back to the free 

 edge of the oesophagus, where the second pair of mesenteries become 

 complete. In stages with twelve mesenteries, however, they have 

 again appeared. The growth of the various tracts of reflected ecto- 

 derm is thus seen to follow in general the order of development of 

 the mesenteries. 



From the above account it will be seen, as Wilson points out, 

 that there is a difference in the mode of formation between the 



