No. 2.] THE EMBRYOLOGY OF LIMULUS. 219 



effected by a breaking down of the wall between them. At 

 first the proventriculus empties directly into the mesenteron, 

 but later (Fig. 72) the inner end of the former becomes drawn 

 out in a slender tube which projects slightly as the "cone" 

 into the intestines. The limits of the two regions, ectodermal 

 and entodermal, at this stage are clearly distinguished by the 

 chitinous cuticle upon the former. 



Proctodeum. — In striking contrast to its development in the 

 Crustacea the proctodaeum in Limulus is late in its appearance 

 and small in extent. As late as Stage I (Fig. 82) it appears 

 merely as a slight inpushing of the ectoderm upon the ventral 

 surface. In Stage L it is wider but scarcely deeper than be- 

 fore (Fig. 81). In still older specimens (Fig. '^'^) the boundary 

 between mesenteron and proctodaeum has broken through and 

 the now more elongate proctodaeum has become thrown into 

 inner folds. At the point of juncture between ectoderm and 

 entoderm there appears, above and below in the section, an 

 enlargement of the lumen of the tube and a second similar 

 enlargement occurs within the entodermal portion of this tract. 

 As to the meanings of these enlargements I have nothing 

 to offer aside from the fact that in connection with them 

 the Malpighian tubules of the Hexapods and the analogous 

 structures of the Arachnids naturally suggest themselves. 



Comparisons. — Almost nothing was previously known of the 

 development of the alimentary tract of Limulus. In 1885 I 

 gave essentially the same account of the formation of the stomo- 

 daeum and the differentiation of the liver-lobes, illustrating 

 both with diagrammatic figures. In the same year Brooks and 

 Bruce in their preliminary paper ('85) describe the entodermal 

 epithelium as arising from the yolk cells in the same way as I 

 have done and they further say, though without any details, 

 that the stomodaeum arises as an ingrowth which at first goes 

 upward and forward and then bends upon itself. Though not 

 specifically the same in words the account given by Kishinouye 

 (9l) of the development of the stomodaeum is easily brought 

 into harmony with the foregoing, and especially interesting is 

 his statement (pp. 79-80) that "As the upper lip grows pos- 

 teriorly, the mouth opening which was at first pre-appendicular 



