THE GERM-BAND AND THE GERM-ENVELOPE. 271 



while the cells belonging to the ventral side, owing to their more 

 rapid division, become crowded close together, assume a prismatic 

 form, and thus constitute a columnar epithelium. The ventral 

 thickening of the blastoderm that has thus arisen, and which, in its 

 extent, represents the first rudiment of the germ-band, was named 

 by Balfour the ventral plate (Fig. 133 A, bp). The invagination 

 of that part of the ventral plate which lies in the median line (g) 

 gives rise to the formation of the lower layer. This invagination, 

 which at a certain stage represents a groove running along the median 

 line for the whole length of the germ-band (Fig. 134 A and B), must 

 be regarded as the gastrula-invagination of the Insecta (for details, 

 see pp. 309 et seq.). The lower layer yielded by it (Fig. 133, 

 B and C, u) then extends beneath the whole of the ventral plate up 

 to the edges of the amniotic fold (Figs. 133 B and 134 C). 



It should be mentioned that the ventral plate from its commencement is not 

 in all cases a uniform structure, but sometimes proceeds from several distinct 

 rudiments. Thus it has been pointed out by F. Schmidt, in connection with 

 Musca, and by Heider in connection with Hydrophilus, that the anterior and 

 posterior ends of the germ-baud appear first, the middle part only developing 

 later. Another originally independent element of the germ-band is afforded in 

 Hyclrojyhilus by the rudiments of the cephalic lobes (Fig. 134 A, k), the inde- 

 pendent origin of which was also observed by Will (No. 97) in the Aphidae. 

 These originally distinct formative centres only secondarily unite to form the 

 common rudiment of the germ-band. 



The lateral delimitation of the germ-band seems determined by the rise of the 

 amniotic fold, and since, when the amnion is complete, it consists of somewhat 

 columnar cells and, even at later stages, owing to its histological character, 

 more nearly resembles the ectoderm of the germ-band than the serosa, some 

 investigators have assumed a closer connection between the amnion and the 

 germ-band. "Will regards the amnion directly as a part of the germ-band, and 

 Graber (No. 30) also derives the amnion from the thickened epithelium of the 

 ventral plate. 



"We have used the term "germ-band" in the usual manner, understanding by 

 it the segmented and already multilaminar embryonic rudiment (consisting of 

 the ectoderm and the lower layer). It is, however, certain that this term may 

 also be applied in a wider sense, as Graber (No. 30) has recently insisted, to 

 the embryonic rudiment of earlier stages, in which segmentation and the forma- 

 tion of the germ-layers has not yet commenced, presupposing that the embryonic 

 rudiment as such is distinctly marked off from the rest of the egg. 



From the time of its origin onwards, the germ-band grows con- 

 tinually in length (Fig. 134-4 to E), and in many cases extends in 

 such a way that it no longer covers only the ventral side of the egg, 

 but its anterior and posterior ends bend round to the dorsal side 

 of the egg. This extension of the germ-band to the dorsal side may, 

 in some cases (Phryganeidae, Chironomus), go so far that the anterior 



