STAGE F. 15 



(fig. 30, d.). It is placed at the level of the eighth somite, 

 and consists simply of a thickening of the dorsal and lateral 

 ectoderm. 



6. This process is shown in its two first stages in figs. 33 and 

 34, st. In fig. 33 the posterior margins of the buccal opening 

 are beginning to grow in beneath the anterior margins ; the 

 same feature being shown more clearly in fig. 34, st. 



7. These pits are caused by an ectodermal invagination 

 which will give rise to the slime glands. 



8. These two perforations (fig. 33, o. s.) are actual perfora- 

 tions leading through the body wall into the body cavity of the 

 third somite (somite of the oral papillae) ; they become the ex- 

 ternal openings of the salivary glands. 



Stage F (fig. 36). — The next stage, which is also found in 

 October in England, is very close to the previous one, and 

 I have only thought it necessary to figure a ventral view of the 

 head (figs. 35, 36) . Fig. 35 is from a specimen slightly younger 

 than fig. 36, in fact from a specimen intermediate between this 

 stage and the previous stage. It has already been referred to 

 as showing the grooves in the brain (c. g.), which first appear in 

 stage E. The main features of interest in this stage relate 

 to the head and anterior somites. 



(1) The lips have become very conspicuous and folded (fig. 36) ; 

 they have extended on to the ventral surface, passing inwards 

 between the jaws and oral papillae, behind the openings of the 

 salivary glands, which they have completely covered up, and 

 finally have united with one another in the median ventral line, 

 so as to form the posterior part of the adult lips. 



Fig. 35 is especially interesting as showing an earlier phase 

 in this growth. In this figure the folds have not yet reached 

 the middle line, and are still very inconspicuous behind the 

 salivary openings (o. «.), which are still exposed. 



(2) The cerebral grooves (fig. 36) have become much deeper, 

 and their opening reduced to a narrow slit, ending behind in 

 the mouth and slightly dilated in front. 



(3) The ingrowth of ectoderm into the mouth-opening is 

 nearly completed in fig. 35. In fig. 36 the mouth-opening has 



