TRACHEATA. 



411 



Peripatus. Hatschek states that in the mandibular segment the median 

 element is absorbed, and that the two lateral cords of that part give rise to 

 the oesophageal commissures, while the sub-cesophageal ganglion is formed 

 from the fusion of the ganglia of the two maxillary segments. 



The prae-oral portion of the nervous system consists entirely 

 of the supra-cesophageal ganglion. It is formed, according to 

 Hatschek, of three parts. Firstly and mainly, of a layer sepa- 



FIG. 189. THREE TRANSVERSE SECTIONS THROUGH THE EMBRYO OF 

 HYDROPHILUS. (After Kowalevsky.) 



A. Transverse section through the larva represented in fig. 187 A. 



B. Transverse section through a somewhat older embryo in the region of one of 

 the stigmata. 



C. Transverse section through the larva represented in fig. 187 B. 



vn. ventral nerve cord; am. amnion and serous membrane ; me. mesoblast ; me.s. 

 somatic mesoblast ; hy. hypoblast (?) ; yk. yolk cells (true hypoblast) ; st. stigma of 

 trachea. 



rated from the thickened inner part of the cephalic lobe on each 

 side ; secondly, of an anterior continuation of the lateral cords ; 

 and thirdly, of a pit of skin invaginated on each side close to the 



