Development of a Termite. 279 



may be so spoken of) is quite thin, being a single layer of 

 cells. These stages, as well as later ones, agree in showing 

 no gastrula invagination, the " under-layer " being formed, 

 as described, rather by a process of delamination or in 

 wandering due to crowding, and the plug being a later 

 secondary formation. 



The facts of the origin of the " under-layer " support 

 Heymons's (10) recent views as to the formation of this layer 

 in the Orthoptera, in as far as they indicate that invagiuate 

 gastrulas may be secondary phenomena among insects. 



As to the origin of the amnion. In the termite it is appa- 

 rently, as has been said, a thickening of the posterior edge of 

 the disk before any trace of a fold can be distinguished in 

 section. When this thickening folds over the ^'isk the amnion 

 is seen to differ in no essential from the rest of the embryonic 

 disk (of course leaving the "under-layer" plug out of con- 

 sideration). 



The enclosure of the germ-disk takes place by the single 

 posterior semicircular fold growing forward to its anterior 

 extremity. Just after the amniotic cavity is closed in this 

 way the amnion is still found to be quite thick and like the 

 upper layers of the disk (see Bruce's figure xliii. of Mantis 

 at this stage (2)). A like similarity has been observed in 

 many insects between the ectoderm of the embryo and the 

 amnion. 



The further growth of the embryo is much like that figured 

 by Graber for Stenohothrus (3). While the anterior end of 

 the disk remains fixed the tail-end grows back over the 

 posterior pole. In this way an embryonic band is formed 

 which makes a cap over this pole. Both ends of the band 

 are at first of the same shape. Soon, however, the anterior 

 extremity spreads out into a broad cephalic area, which has 

 reached its greatest extent by the time the posterior end of 

 the band has pushed up about one third of the dorsal surface 

 of the egg. Segmentation now sets in — the antennary 

 (postoral), mandibular, first and second maxillary, and first 

 thoracic segments appearing almost simultaneously. 



There are no macrosomites, as in Stenohothrus (Graber). 

 The remaining thoracic and abdominal segments are added 

 successively from before backward, as the band grows still 

 further toward the anterior end of the egg. The labrum 

 appears as a median unpaired fold over the mouth. 



Sections of these early stages of the elongating embryonic 

 band show that the " under-layer " does not extend anteriorly 

 beneath the ectoderm, which has spread out anteriorly over 

 the yolk to form the cephalic lobes. Posteriorly, however, 



20* 



