Lateral Tracts 167 



between the approximated ends of the body. Between 

 the two a clear iluid can be detected with some difficulty. 



The original motive, so to speak, of the amnion of 

 insects can only be guessed at. It may be rudimentary 

 (Muscidae), or wanting altogether (Poduridae, Cecido- 

 myia, &:c.), and its relation to the yolk varies greatly 

 in different insects. One special use of the amniotic 

 folds may be noted. In some insect-eggs (Chironomus, 

 &c.) ih.Q embryo is long and peripherally coiled, so that 

 the head and tail nearly meet ; the intervening extra- 

 embryonic blastoderm is naturally short. In a later stage 

 the embryo straightens itself, so that the head gets to 

 one end of the egg, and the tail to the other. This 

 straightening is greatly facilitated by the extended folds 

 of the blastoderm. Whatever circumstances may have 

 led to the first development of an amnion, it seems to be 

 now a protective structure, protecting the delicate body 

 from friction. In a late stage the serosa of the Chiro- 

 nomus- egg has been found to be retracted into the dorsal 

 surface of the embryo, and to be there incorporated with 

 the yolk '. The amnion persists as a dead and shrivelled 

 membrane, which can often be seen within an egg from 

 which the larva has escaped ^. 



The ventral plate is from a rather early stage, when the Lateral 

 amniotic folds are beginning to form, marked along the 

 middle line of its free surface by a much more conspicuous 

 and wider groove than that of the ventral fold already 

 described. This runs along the body from the tail-end 

 to the junction of the future head and thorax, where it 

 ends by forking. The groove marks out a pair of thicken- 

 ings, the lateral fi'acfs (Kehmciils-fe), which are promi- 

 nent features of the embrj^o during the middle stages of 

 its growth. 



At the time when the lateral tracts appear the embryo Rotation of 



embryo. 

 ' Graber, 1888, p. 34. - Id. (loc. cit.). 



