No. 2.] THE EMBRYOLOGY OF LIMULUS. 1 99 



it impossible to say whether a portion is left in the ventral 

 region of each segment. That a portion does so persist in the 

 fifth segment will appear later. 



Until after the splitting of the chorion only eight pairs of 

 schizocoelia are produced, there being a pair for each^ (Fig. 46) 

 cephalothoracic and for the two anterior abdominal somites. 

 At first these cavities are flat, and broader than long ; the first 

 pair, however, rapidly elongates and sends a diverticulum 

 forward beneath the brain on either side of the oesophagus, 

 into the preoral region. At first all of the cavities are distinct, 

 and their walls epithelial in character, but soon it becomes 

 difficult to follow their fate with certainty since a secondary 

 splitting of the mesoderm soon produces a large number of 

 anastomosing lacunar cavities {e.g. Figs. 53 bs., 59 lac, etc.) 

 connected later with the vascular system and developing into 

 the so-called "body-cavity" of the Arthropod, the existence of 

 which much confuses the sections. 



With the growth at first laterally, then dorsally and medially, 

 of the two halves of the mesoderm, a portion (if not all) of 

 the ccelom in somites II, III, IV, VI and VII, and a part of 

 that in somite V, is carried towards the dorsal median line of 

 the embryo, where, in the latest stages I have studied, the 

 cavities, now run together, persist as a longitudinal tube 

 (Figs. 63-67) beneath the pericardial sinus, on either side of 

 the heart and its anterior arterial prolongation. Posteriorly 

 this paired cavity does not at any time extend further back 

 than somite VIII or IX. Whether in any of the somites all 

 of the coelom is thus carried to the dorsal surface I am unable 

 to say, while I have not followed the fate of the coelom of 

 somite I. According to Kishinouye the anterior coelomic 

 pouches are pushed inward with the advancing stomodaeum, and 

 hence give rise to the splanchnoplure of the oesophagus and 

 proventriculus. In somites I-IV, and also in somite VI, I am 

 unable to recognize any ventral cavity as distinctively coelom 

 much later than the time when motion is seen in the append- 

 ages. In the posterior abdominal somites the coelom persists 



1 According to Kishinouye there is no coelom in somites II, III, IV of the 

 Japanese Limulus. 



