240 THE EMBRYOLOGY OF THE BLOW-FLY L\ THE EGG. 



observations indicate that it first appears on the ventral surface, 

 or near the posterior pole of the egg, and gradually extends 

 over it, or that it is at first a small cup-like blastoderm buried 

 in the interior of the yelk, as in the Blow-fiy. 



Whatever view may be held as to the manner in which the 

 blastoderm originates, all observers agree that in the egg of the 

 Blow- fly, two or three hours after impregnation, it forms a 

 single layer of columnar epithelial cells, enclosing the whole 

 food-yelk. This is the blastula stage. The cells, however, near 

 the posterior pole in the region corresponding with the polar 

 cells are smaller than the rest, and a few of the segmentation 

 cells, resulting from the division of the polar cells, apparently 

 remain adhering to its surface. 



The Gastrula Stage. — L. Will [108] remarks on this subject : 

 'There are in the development of insects two stages which may 

 be equally regarded as gastrulation. The first, which I shall 

 term Gastrula No. i, occurs at a very early period of develop- 

 ment. Part of the cells derived from segmentation arrive at 

 the periphery of the yelk, and form the ectoderm ; others remain 

 in the yelk, and form the endoderm. The second, my Gastrula 

 No. 2, occurs at a much later period, after the whole egg is 

 enclosed in the blastoderm, and consists in a sinking-in of the 

 middle of the primitive band, so as to form a long median 

 fissure, the median groove {Kcimfurchc, or Mcsodermrinnc).' 



I am averse to the use of the term gastrula in either of the 

 senses in which it is used by Will, as neither one nor the other 

 stage represents the gastrula of either an Echinoderm, Mollusc, 

 or Vertebrate in any sense whatever, and the use of the term 

 indiscriminately for either can onl}^ lead to serious miscon- 

 ceptions. 



True gastrulation gives rise to the formation of an archenteron, 

 bounded by hypoblast. If we compare the stage called by 

 Will ' Gastrula No. i ' with the corresponding stage of the 

 Echinoderm embryo it is clear that the so-called hypoblast is 

 the mesenchyme. 



With regard to Will's 'Gastrula No. 2 ' in the Blow-fly embyro, 

 a ventral invagination of the epiblast certainly occurs ; but this 



