The Development of Theridium. 335 



sacs for the head lobe. Schiinkewitsch (1887) described the rostrum 

 ("levre superieure") arising as a pair of tubercles, basally con- 

 tiguous, just anterior to the mouth; a similar pair of tubercles aris- 

 ing later (''levre inferieure") border the mouth posteriorly; the 

 rostrum forms by coalescence of these two pairs. The mesoblast of 

 the cephalic lobe is continuous with that of the chelicera, but later 

 separates from it. All the cephalothoracic segments appear simul- 

 taneously except the cheliceral. Jaworowski (1891.) described in 

 Trochosa as "antennae" a pair of small unjointed prominences imme- 

 diately anterior to the chelicera, but as seen on surface views only ; 

 he did not decide whether the ''Oberlippe" (rostrum) is paired in 

 origin. In this and a later paper (1892) he brings evidence to show 

 the cleft, crustacean, nature of arachnid extremities. Kishinouye 

 (1894) found the mesoblast of the head lobe continuous with that of 

 the chelicera, and though a pair of coelomic spaces develop for the 

 chelicera and two pairs later for the head lobe, the wall of all of 

 them remains continuous. Pokrowsky (1809, Pliolcus) described 

 briefly, and from a surface view only, two jjairs of "Kopfhocker" 

 at a stage when the limbs are jointed; the anterior lie above the 

 antero-lateral vesicle, and the posterior (which he compares with 

 the "antennae" of Jaworowsky) somewhat ])osterior to them; he 

 figured the rostrum as bilobed. Pappenheim (1903, Dolomedes) 

 described the coelomic sacs of the cephalothorax, and found that of 

 the cephalic lobe to be from the start separate from that of the 

 chelicera ; the "upper lij)" and "lower lip" are differentiations of an 

 unpaired prominence in front of the mouth; Pappenheim opposed 

 the existence of the prcoral extremities of Jaworowsky and Pokrow- 

 sky. Wallstabe (1908) finds there is a separate head coelom; and he 

 also discovered a special egg tooth near the base of each pedipalp. 



The special points in which I have compared the results of other 

 investigators are (1) the order of appearance of the cephalothoracal 

 appendages and segments, (2) the matter of an independent meso- 

 blast in the cephalic lobe, and (3) the matter of appendages on the 

 cephalic lobe. All observers seem to agree that the cheliceral seg- 

 ment arises latest in a postoral position by se})arating from the head 

 lobe, and that at reversion the chelicera move anterior to the mouth. 



