PCECILOPODA. 



535 



flexed in the middle in such a way that their ends become directed towards 

 the median line (fig. 245 B). The body of the embryo (fig. 245 B) is 

 now distinctly divided into two regions the cephalothoracic in front, and 

 the abdominal behind, both divided into segments. 



FIG. 245. THREE STAGES IN THE DEVELOPMENT OF LIMULUS POLYPHEMUS. 

 (Somewhat modified from Packard.) 



A. Embryo in which the thoracic limbs and mouth have become developed on 

 the ventral plate. The outer line represents what Packard believes to be the amnion. 



B. Later embryo from the ventral surface. 



C. Later embryo, just before the splitting of the chorion from the side. The full 

 number of segments of the abdomen, and three abdominal appendages, have become 

 established ; m. mouth ; I IX. appendages. 



Round the edge of the ventral plate there is a distinct ridge the 

 rudiment of the cephalothoracic shield. 



With the further growth of the embryo the chorion becomes split 

 and cast off, the embryo being left enclosed within the inner membrane. 

 The embryo has a decided ventral flexure, and the abdominal region 

 grows greatly and forms a kind of cap at the hinder end, while its 

 vaulted dorsal side becomes divided into segments (fig. 245 C). Of these 

 there are according to Dohrn seven, but according to Packard nine, of 

 which the last forms the rudiment of the caudal spine. 



In the thoracic region the nervous system is by this stage formed as 

 a ganglionated cord (Dohrn), with no resemblance to the peculiar cesopha- 

 geal ring of the adult. The mouth is stated by Dohrn to lie between the 

 second pair of limbs, so that, if the descriptions we have are correct, it must 

 have by this stage changed its position with reference to the appendages. 

 Between the thorax and abdomen two papillae have arisen which form the 



