556 



TEXT-BOOK OF ENTOMOLOGY 



The gnathal or post-antennal segments at first bear but a small 

 part in completing the tergal region of the head, but shortly before 

 hatching the mandibles and their muscles enlarge, giving fulness 

 to the upper and back part of the head. 



am- 



i. Dorsal closure and involution of the embryonic membranes 



In most other Arthropoda (Crustacea, Arachnida, Myriopoda, etc.) 

 development goes on by the formation of a so-called primitive band, 



but without the appearance of peculiar 

 embryonic membranes. The outer surface 

 of the entire egg becomes, then, in part 

 covered by the band-like embryonic germ, 

 and partly by a portion of the blastoderm 

 which remains unchanged. The dorsal 

 region is formed by the widening and 

 spreading of the primitive band over the 

 greater part of the surface of the egg, 

 while the area of the unchanged section 

 of the blastoderm continually becomes 

 more restricted. It is generally accepted 

 that the latter is concerned in the dorsal 

 closure, because, together with a histo- 

 logical transformation, it becomes in- 

 volved in the formation of the ectoderm 

 of the primitive band. 



FIG. 531. Schematic figure of 

 the formation of the dorsal tube 

 by invagination of the dorsal plate 

 (transformed serosa) ; following 

 after stage Fig. 520, f, and Fig. 521, 

 D ; am, amnion (now forming the 

 provisional dorsal closure) ; r, dor- 

 sal tube, whose cells are already 

 breaking away. After Korschelt 

 and Heider. 



A similar form of retrograde structure possi- 

 bly occurs in the embryos of Poduridse, in which 

 a dorsal organ has been observed to develop in 

 an early embryonic stage, which bears some 

 relation to the cuticula enveloping the embryo, but whose significance is in 

 general rather obscure. 



In most insects the relations are more complicated, since in such cases, the 

 amnion-folds rise on the edges of the primitive band and of the unchanged sec- 

 tion of the blastoderm, whose retrograde development is intimately connected 

 with the closure of the back. 



A very simple case of dorsal closure, but which certainly is not a primitive 

 one, occurs in Muscidae and certain other Diptera whose amnion-folds are de- 

 veloped in a rudimentary way. In this case (according to Kowalevsky and 

 Graber), the amnion-folds become smoothed out again. Amnion and serosa 

 become then a simple epithelium, which throughout corresponds to the unmodi- 

 fied type of blastoderm of Crustacea, Arachnida, and Myriopoda, and here seems 

 to share in the formation of the back. More complicated and very manifold 

 relations of dorsal closure and involution of the embryonal membranes occur 

 in other insects, of which Korschelt and Heider distinguish four different types : 



