COMPLETION OF THE DORSAL PART OF THE EMBRYO. 



303 



an. 



am — 



egg is then covered partly by the band-like embryonic rudiment and 

 partly by the unmodified blastoderm. The dorsal part of the embryo 

 is there formed by the continuous broadening of the germ-band which 

 by its growth, extends over the greater part of the surface of the egg, 

 the region covered by unmodified blastoderm becoming more and 

 more circumscribed. It is as a rule assumed that the latter takes 

 part in the closing of this dorsal region by being transformed histo- 

 logically to form the ectoderm of the germ-band. It is possible that 

 in these forms also part of this blastoderm 

 gradually degenerates. We have (Vol. ii., 

 p. 150) conjecturally referred the formation 

 of the so-called dorsal organ of certain Crus- 

 tacea to such a process of degeneration. A 

 similar method of development of the dorsal 

 part of the embryo perhaps also occurs in the 

 Poduridae, in which a dorsal organ is found 

 which develops in the early embryonic stages, 

 and is connected with a larval cuticle that 

 envelops the embryo (Lemoine, Xo. 51), but 

 in other respects its significance is somewhat 

 obscure (p. 268). In most insects the process 

 is more complicated, in so far as an amniotic 

 fold arises at the junction of the germ-band 

 with the undifferentiated part of the blasto- 

 derm, the degeneration of this fold being 

 intimately connected with the completion of 

 the dorsal surface of the embryo. 



A very simple case of the formation of the 

 dorsal region in the embryo which, however, 

 we can certainly not regard as primitive, is 

 found in the Mus<-idae and a few other Diptera 

 in which the amniotic fold is incomplete 

 (p. 284). Here (according to Kowalevskv, 

 No. 49, and Graber, Xo. 28) the amnion is simply flattened out 

 The amnion and the serosa then together form a simple 



Fig. 14S. — Diagram of the 

 development of the dor- 

 sal tube through invagi- 

 nation of the dorsal plate 

 (transformed serosa). 

 Succeeding the stage de- 

 picted in Figs. 13S C and 

 140 D. am, amniotic 

 fold (now forming the 

 provisional dorsal in- 

 tegument) ; r, dorsal 

 tube, which is already 

 commencing to disinte- 

 grate. 



again. 



epithelium which corresponds to the unmodified part of the blasto- 

 derm in the Crustacea, Arachnida, and Myriopoda, and here also 

 seems to take the same part in the development of the dorsal ectoderm. 

 More complicated and very varied methods of formation of the dorsal 

 region and of the involution of the embryonic envelopes are found in 

 the other Insecta, the four following types being distinguishable. 



