FORMATION OF MEMBRANES AND PRIMITIVE BAND. 245 



consideration of the probability of any possible morphological 

 connection between the embryonic form of the Insect and the 

 Lamellibranch or Ascidian to be discussed by those who have 

 made a special study of the MoUusca. 



The Primitive Band {Kciimtrcif) may be described as a 

 thickened band which usually appears on the ventral surface of 

 the blastula. In the Lepidoptera a discoid thickening of the 

 epiblast first appears, the germinal area, which, according to 

 Graber [115], afterwards becomes a strap-shaped primitive 

 band, wider at its anterior end. This change occurs as it 

 sinks into the yelk. In many insects the primitive band occu- 

 pies only a small portion of the ventral surface of the blastula ; 

 in others it extends over the whole ventral surface, and in 

 some Diptera, as in the Muscidae and Chironomus, at one 

 period it surrounds the yelk ; its anterior and posterior ends, if 

 we include the large procephalic lobes, almost meeting on the 

 dorsal surface of the yelk. The width of the primitive band is 

 also subject to considerable variation. The lateral edges of the 

 dorsal and ventral portions in the Blow-fly are only separated 

 by a narrow chink, through which the food-yelk is seen as a 

 dark line. 



It is usual to designate the primitive band in Insects as 

 endolecithal when it sinks into the yelk, as in the Lepidoptera, 

 and epilecithal when it remains upon its surface, as in the 

 Coleoptera. In many insects it is partly endolecithal and 

 partly epilecithal. Whether the primitive band is endo- or epi- 

 lecithal, the membranes have the same relation to the yelk, 

 only in the latter case the serosa and amnion are so closely 

 applied to each other that the yelk does not enter the inter- 

 amnial cavity (see PI. XII.). 



The origin of the primitive band in the Fly-embryo, according 

 to the view I have adopted, is somewhat different, and closely 

 resembles the manner in which the primitive band is formed 

 according to Will [108], in the Aphides. It is developed from 

 the cells which form the thick-walled amnial tube, only a few 

 of which become flattened out to form the amnion. The 

 cells of the primitive band are far smaller than those 



