No. 2.] THE EMBRYOLOGY OF LIMULUS. 213 



noplure, while the somatoplure gives rise to the alary muscles. 

 If this be so it is an important point of difference between 

 Limulus and the Arachnids. Schimkewitsch further describes 

 the pulmonary veins arising as outgrowths from the pericardium, 

 the lateral arteries as outgrowths (Ausstiilpulgen) from the 

 heart itself. 



Alimentary Tract. 



The alimentary tract of Limulus, like that of all Arthropods, 

 consists of three divisions ; stomodaeum and proctodaeum, of 

 ectodermal origin, and mesenteron (including the "intestine" 

 and "liver"), derived from the entoderm. These parts are 

 easiest considered in connection with each other. 



Mesenteron. — The separation of the entoderm from the 

 ecto-mesoderm by delamination was described (p. 46) in the 

 first part of the present article. As will be remembered, I 

 regard the whole of the nucleated yolk after that separation 

 as the true entoderm. From the time of differentiation of 

 this layer until the first molt there is but slight histological 

 change in the region of the midgut and its diverticula aside 

 from a slight increase in number and consequent decrease in 

 size of the yolk (= entoderm) cells. There is, however, a 

 very considerable change in the shape of the entoderm which 

 may be summarized as follows : — 



When the entoderm is first separated from the rest of the 

 Qgg, it is, like the Qgg, spherical in outline. It then becomes 

 gradually flattened {cf. Fig. 82) and more and more ovoid in 

 outline, viewed from above, corresponding in this with the 

 changes in shape of the embryo. As a result there may 

 soon be distinguished a large semicircular mass of entoderm 

 in the cephalothorax and a smaller, more cylindrical portion 

 in the abdomen. Coincidently with this change in outline 

 the beginning of the differentiation of midgut and "liver" 

 occurs. As the mesoderm extends itself peripherally from 

 the median ventral line of the embryo, it gives rise to slight 

 intersegmental ridges, the septa of Balfour. Until this cen- 

 trifugal growth reaches its extreme these septa are slight in 

 extent, but as it attains the margin of the carapax and, turning 



