310 INSECTA. 



rudiment of the entoderm and the mesoderm (Fig. 158 A-C, p. 321). 

 A similar condition was found by Kowalbvsky in Apis, in the 

 Lepidoptera, and in a few other, forms. This furrow must be 

 regarded as the blastopore of an unusually long gastrula-depression, 

 extending along the whole ventral surface as far as the point at 

 which, later, the proctodaeum develops, and the edges of the furrow 

 must be regarded as the lip of an exceedingly long blastopore. The 

 tube which has arisen in HydropMlus by the closing of this furrow 

 may be claimed as the archenteron. 



The first rudiments of the gastrula-furrow are, in the Insecta, 

 yielded by two folds running longitudinally in the thickened ventral 

 plate one on either side of the median line (Fig. 154, /, p. 313). 

 These folds cut off a middle region of the ventral plate, the so-called 

 middle plate (m) from the lateral plates (s). As the middle plate 

 bends in and becomes grown over by the lateral folds, which mark 

 the edges of the blastopore, the gastrula-depression is formed (Fig. 

 158 A, r, p. 321), the development of which causes the middle plate 

 to become the lower or inner layer of the germ-band. The ectoderm 

 of the germ-band is then derived from the lateral plates. The fusion 

 of the edges of the blastopore, through which the closing of the 

 archenteric tube is brought about, occurs latest in its most anterior 

 region, at a part of the germ-band corresponding to that at which 

 later the stomodaeal invagination develops. 



In HydropMlus, the gastrula-furrow develops in a way differing somewhat 

 from that which usually prevails, as the middle part of the furrow here appears 

 somewhat retarded in its development, while, in the anterior and posterior 

 regions, the lips approximate earlier. This growth affects the outline of 

 the blastopore, which at a certain stage is flask-shaped (Fig. 134 A, p. 270), 

 the bulging of the flask corresponding to the part of the germ-band which is 

 retarded in its development. 



During the invagination of the middle plate and its transformation 

 into the archenteric tube it becomes modified histologically (Fig. 158 

 A and B, p. 321). Whereas it primarily consisted of a columnar 

 epithelium, which in the further course of development becomes 

 multilaminar, the individual cells, pressed together, being wedge- 

 shaped, the cells in later stages become more and more cubical or 

 irregularly polygonal (Fig. 158 B), and also show a less regular 

 arrangement. At the same time the archenteric tube becomes com- 

 pressed dorso-ventrally. While it thus broadens out laterally under 

 the lateral plates, its originally circular lumen passes into a horizontal 

 slit, which in Hydrophilus long remains recognisable as the boundary 

 between the two parts of the lower layer (Heider, No. 38). 



