546 



INSECTA. 



folds are then formed ; these lead to the formation of the embryonic 

 membranes, which are so characteristic of insect development. In 



. 455. Development of the embryo of Hydrophilus piceus (after Kowalevski). a, Shield- 

 like ventral plate with raised edges, b, The edges are already growing together in the 

 middle, c, The groove is almost entirely closed, d, The tail fold of the embryonic 

 membranes has grown over the posterior end of the closed groove and is gradually 

 extending forward ; Am, Amnion. e, The embryonic membranes have almost entirely 

 grown over the embryo, f, The embryonic rudiment beneath the completely closed mem- 

 branes ; with seventeen primitive segments : Kl, Procephalic lobes ; A, antennae, g, The 

 ventral plate extends along the whole length of the ventral surface. The bi-lobed upper lip 

 is present, also the antennae, (A) the jaws, and the first rudiments of the legs; rudimentary 

 appendages are present on the seventh segment as prominences. On the abdominal 

 segments there are round invagiuations, the first rudiments of tracheae ; there is a 

 longitudinal groove from mouth to anus, h, The ventral plate covers the whole ventral 

 surface of the ovum ; the openings of the invaginations (stigmata) have become small ; 

 rudimentary extremities are still present on the first abdominal segment. The ganglia 

 of the ventral chain have appeared, i, Viewed from the dorsal surface the so-called 

 dorsal plate has closed up to a tube; Oe is its opening, k, The embryo just before 

 hatching seen from the ventral side. 



Hydropldlus these folds grow together over the ventral plate from 

 behind forwards, and fuse with one another, so as to give rise to an 



