310 INSECTA. 



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



A similar condition was found by Iyowaleysky 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 Hydrophilus 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 Hydrophilus, 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). 



