330 



INSECT A. 



slight consideration of the mode of formation of the membranes, or 

 an inspection of the figures illustrating their formation, makes it at 

 once clear that the yolk can pass in freely between the amnion and 

 serous envelope (vide fig. 181). At the hind end of the embryo this 

 actually takes place, so that the ventral plate covered by the amnion 

 appears to become completely imbedded in the yolk : elsewhere the two 

 membranes are in contact. At first (fig. 176) the ventral plate occupies 

 but a small portion of the ventral surface of the egg, but during the 

 changes above described it extends over the whole ventral surface, and 

 even slightly on the dorsal surface both in front and behind. It 

 becomes at the same time (fig. 179) divided by a series of transverse 

 lines into segments, which increase in number and finally amount 

 in all to seventeen, 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 rudi- 

 ments at the close of the last stage, but soon 

 become much more prominent (fig. 180 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. 180 A, give rise 

 to the tracheae; and the nervous system is formed as two lateral 

 epiblastic thickenings, one on each side of the mid-ventral line. 

 These eventually become split off from the skin; while between them 

 there passes in a median invagination of the skin (fig. 189 C). The 

 two nervous strands are continuous in front with the supra-oesophageal 

 ganglia, which are formed of the epiblast of the procephalic lobes. 

 These plates gradually grow round the dorsal side of the embryo, and 

 there is formed immediately behind them an oral invagination, in front 

 of which there appears an upper lip (fig. ISO, Is). -A proctodreum is 

 formed at the hind end of the body slightly later than the stomo- 

 daeum. The mesoblast cells become divided into two bauds, one on 

 each side of the middle line (fig. 189 A), and split into splanchnic and 



FIG. 179. EMBRYO 

 OF HYDROPHILTJS PICEUS 

 VIEWED FROM THE VEN- 

 TRAL SURFACE. (After 

 Kowalevsky.) 



pc.l. procephalic lobe. 



