THE NYMPHOID STAGE OF THE EMBRYO. 261 



it by a transverse line. The mandibles at this period lie in the 

 mouth-cleft, and sink more and more deeply into it. It is 

 remarkable that the fore-head takes no part in the formation of 

 the larval head, but is entirely invaginated ' [2, p. 64]. 



On this point Weismann adds : ' After the invagination of 

 the fore-head within the mouth, both pairs of maxillae grow 

 forwards ; especially the anterior, which become considerably 

 enlarged, project beyond the fore-head, and draw the pro- 

 cephalic lobes together, whilst the latter become smaller and 

 smaller, and unite with the maxillae [2, p. 67] . 



With the exception of the last statement, Weismann's account 

 appears to me to be quite correct. Weismann believed that the 

 dorsal sensory organs on the maxillae of the larva represent 

 antennae, and hence he supposed that the procephalic lobes 

 unite with the rudimentary maxillae. 



Van Rees [147] confirms by his views and observations an 

 opinion which I expressed as early as 1872 [142], that the pro- 

 cephalic lobes are invaginated to form the cephalic discs, 

 although Graber is led to a different conclusion, as he asserts 

 that these discs in the fly embryo are at first composed of a 

 single layer of cells [114]. Certainly, his drawings lead to such 

 a conclusion, but in the face of the evidence afforded by my 

 sections and the observations of Van Rees, I cannot regard 

 Graber's sections as conclusive. It is possible that certain 

 sections, through the thick edges of the discs, would present 

 these appearances, but I think it probable that further investi- 

 gations will show that the disc cavity which exists at the earliest 

 stage of development is never entirely obliterated. Graber's 

 sections are apparently made from embryos in a more 

 advanced condition than mine, and it may be that at this 

 stage the disc cavity is relatively small, and that the edges 

 of the disc form considerable solid outgrowths, over which 

 the cavity subsequently extends ; but on this point further 

 observations are needed. In newly-hatched larvae the disc 

 cavities are certainly distinct. 



