TRACHEATA. 



399 



FIG. 179. EMBRYO OF 

 HYDROPHILUS PICEUS 



VIEWED FROM THE VEN- 



TRAL SURFACE. (After 

 Kowalevsky.) 



pc.L procephalic lobe. 



by a series of transverse lines into segments, which increase in 

 number and finally amount in all to seven- 

 teen, not including the most anterior section, 

 which gives off as lateral outgrowths the 

 two procephalic lobes (pc.l). The changes 

 so far described are included within what 

 Kowalevsky calls his first embryonic period; 

 at its close the parts contained within the 

 chorion have the arrangement shewn in fig. 

 178 B. The whole of the body of the 

 embryo is formed from the ventral plate, 

 and no part from the amnion or serous 

 envelope. 



The general history of the succeeding 

 stages may be briefly told. 



The appendages appear as very small 

 rudiments at the close of the last stage, but 

 soon become much more prominent (fig. 

 1 80 A). They are formed as outgrowths of both layers, and 

 arise nearly simultaneously. There 

 are in all eight pairs of appendages. 

 The anterior or antennae (at) spring 

 from the procephalic lobes, and 

 the succeeding appendages from 

 the segments following. The last 

 pair of embryonic appendages, 

 which disappears very early, is 

 formed behind the third pair of 

 the future thoracic limbs. Paired 

 epiblastic involutions, shewn as pits 

 in the posterior segments in fig. 

 1 80 A, give rise to the tracheae; 

 and the nervous system is formed 

 as two lateral epiblastic thicken- 

 ings, one on each side of the mid- 

 ventral line. These eventually be- 

 come split off from the skin ; while 

 between them there passes in a 

 median invagination of the skin 



FlG. 1 80. TWO STAGES IN THE 

 DEVELOPMENT OF HYDROPHILUS 



PICEUS. (From Gegenbaur, after 

 Kowalevsky.) 



Is. labrum ; at. antenna ; md. 



