246 THE EMBRYOLOGY OF THE BLOW-FLY IX THE EGG. 



which form the amnial tube, and evidently result from their 

 subsequent division, whilst the greater part of the external 

 la3-er of the blastoderm becomes converted into the serosa. 

 Graber gives a ligure of the primitive band and membranes in 

 H}lotoma, one of the Saw-flies, in which precisely the same 

 relations exist [115, Fig. 131]. 



Structure and Segmentation of the Primitive Band.^The primi- 

 tive band has not been investigated at an early period of 

 development in the Blow- fly embryo, so that I am only able 

 to describe its structure from observations on other insects. 

 It consists of a thickening of the epiblast, the result of a 

 proliferation of its cells, but there is also a large development 

 of parablastic elements, which penetrate between the deeper 

 epithelial layers and form a sub-epiblastic stratum. These 

 parablastic elements have hitherto been confounded with the 

 mesoblast (PL XII., Fig. 7). 



The primitive median furrow and the stomodaeal pit appear at 

 a very early period of development, and immediately after- 

 \\ards the parablastic cells, which previously were evenly dis- 

 tributed on either side of the median furrow, break up into 

 groups corresponding with the body segments. The cause of 

 this separation of the parablast into groups of cells appears to 

 me to be the rapid proliferation of the epiblast at the points 

 where the nerve-ganglia are subsequentlv developed (PL XII., 

 Fig. 6). 



The process of segmentation has not been observed in the 

 Blow-fly, or, indeed, in any of the Muscidae at this early period. 

 It evidently occurs with great rapidity, but there is no reason 

 to suppose that it differs from that of other insects in which 

 the numerous observations b\- various observers are all sus- 

 ceptible of the interpretation given above. The external layer 

 of epithelium derived from the proliferating epiblast forms 

 the hypodermis ; the deeper cells develop into the primitive 

 ganglia. The connective tissue and supporting investment of 

 the ganglia, with the tracheae, fat bodies and blood, originate 

 from the parablastic layer. 



Contraction of the Primitive Band. — During the segmentation 



