ORIGIN OF THE ARCHENTERON. 251 



from the last polar cells which lie within the invaginated 

 archenteron, is a matter which I cannot settle. 



Will [108] describes and figures a group of cells in the 

 aphis embryo, which he terms the sexual rudiment. It appears 

 to me to be in reality the hypoblast, and to represent the polar 

 cells of the Diptera. 



Formation of the Somatopleural Epiblast.— During the de- 

 velopment of the archenteron the epiblast of the primitive 

 band extends laterally and grows upwards over the yelk, 

 separated from it by the thick layer of parablastic cells, in 

 which the rudimentary ganglia of the ventral chain are im- 

 bedded (PI. XIV., Fig. 2), so that the embryo becomes boat- 

 shaped (Fig. 40). 



Fig. 40.— An embryo of the Blow-fly removed from the egg before the somato- 

 pleural epiblast closes over the dorsal surface of the yelk. 



It will be seen by the figure that before the segmentation of 

 the body is apparent externally, the great procephalic lobes 

 meet upon the dorsal surface of the head, and the fore-head 

 appears as a vesicle on its ventral surface, in front of the 

 stomodaeum. The rudiments of the mandibles and maxilla 

 are also present as thick folds on either side of the primitive 

 mouth. At this stage the dorsal surface of the yelk is only 

 covered by parablast, which bounds a cavity, regarded by 

 Cholodkowsky as representing a segmentation cavity. 



Origin of the Mesoblast. — At a very early period of develop- 

 ment a pair of involutions occur, one on either side of the 

 primitive hypoblast (PI. XIII., Fig. 5, and PI. XIV., Fig. 2). 

 These I regard as the coelomic sacs. They afterwards become 



