No. 2.] THE EMBRYOLOGY OF LIMULUS. 21 5 



lobes into lobules (Figs. 34, 35) resulting in the adult condi- 

 tion. 



At the anterior end of the body an ingrowth ^ similar to 

 the septa carries back the anterior end of the intestine, and 

 intervenes to separate the first pair of lobes from each other. 

 With this ingrowth this pair of lobes, which at first were at 

 right angles, come to lie parallel to the principal axis of the body. 



The central unsegmented part of the yolk which remains 

 after the differentiation of the "liver" forms the "intestine" 

 of the adult. It extends from the point of the first appearance 

 of the stomodasum back to the posterior end of the body. 



Until after the first molt after hatching the entoderm retains 

 the same histological characters which it had at its first differ- 

 entiation. It is a mass of yolk without lumen and is divided 

 into a number of polygonal cells with clearly marked cell walls 

 and central nuclei.^ Excepting in a slight difference in size, 

 it is impossible to distinguish histologically between the ento- 

 derm cells of Stages C and I. That some change does occur 

 in the interval, of a chemical rather than of a histological 

 character, is shown by the fact that while in the earlier stages 

 the yolk is very difficult to section, in the later it cuts as 

 readily as any other tissue of the body. 



After the molt which produces the adult form (Stage L) the 

 histiogenesis of the epithelium of the midgut and its diverticula 

 begins. It appears first in the intestine and later in the liver ; 

 and in the intestine it is first seen at the anterior end (Fig. 81). 

 From the study of numerous sections {cf. Figs. 81, 85, 88) the 

 process is clearly seen to be a direct conversion of the yolk- 

 cells into the epithehal lining of the mesenteron. In Fig. 85 



a sagittal section through the junction of stomodaeum and 



mesenteron at early stage L — the entoderm cells, en, near the 



1 For clearness this and the lateral septa are considered as ingrowths, but they 

 are to a large extent outgrowths as well, since the margin of the carapax is 

 farther removed from the median line in the later than in the earlier stages, and 

 it is coincidently with this change in the relative position of the margin of the 

 body that the septa are developed. This is even more marked in front than at 

 the sides of the body. 



2 In my figures the yolk is represented as solid, neither cell walls or nuclei 

 being shown. They are, however, very distinct in all of my preparations. 



