248 THE EMBRYOLOGY OF THE BLOW-FLY IN THE EGG. 



appears to amount to this— that the archenteron is entirely 

 parablastic ; and such an hypothesis is without doubt com- 

 pletely at variance with all that is known of the develop- 

 ment of the Metazoa generally. 



The view which I adopt is that the so-called proctodaeum m 

 Insects is really a gastrulation, and is the true archenteron. 



I was first led to this opinion by the relation of the coelomic 

 sacs with the so-called proctodseal opening, which I regard as a 

 true blastopore ; and I shall show that a great number of facts 

 are in accord with my' hypothesis, which is still further con- 

 firmed by the discovery of what I take to be a true proctodaeal 

 involution of the epiblast, near the posterior extremity of the 

 ventral surface of the blastula (Fig. 3. P- 8). 



It has long been known that in Chironomus the polar cells 

 of Weismann travel forwards along the dorsal wall of the ovum 

 during the formation of the primitive band, and then sink into 

 the yelk. Balbiani supposed that these cells form the sexual 

 glands,* an opinion which has received much support. 



In the Fly embryo the polar cells, by continued division, 

 form a disc of small cells at the posterior pole of the ovum~my 

 hypoblast— upon which a few spherical cells are sometimes 

 seen to be adherent to the blastoderm. These, like the polar 

 cells in Chironomus, travel forwards on the dorsum of the yelk, 

 and sink into my archenteron at the dorsal extremity of the 

 primitive band. Voletzkow has found them in the interior of 

 the so-called proctodaeal involution [111] . 



The involution of the blastoderm in the dorsal region of the 

 embryo is preceded by the formation of a group of large cells 

 at the spot previously occupied by the polar cells (PI. XIII., 

 Figs. 5 and 6). These form my primitive archenteron, and I 

 term the orifice of invagination the blastopore. As the primitive 

 band becomes shortened, the blastoderm between its hinder end 

 and the blastopore assumes the same characters as the arch- 

 enteric invagination (Fig. 38, p. 2). The edges of this part of the 

 blastoderm become curved upwards, so that a groove is de- 

 * Balbiani. E. G., ' Contribution h I'dtude de la formation des organes 

 sexuels chez les insectes,' Recueil. Zool. .Suisse, Bd. ii., 1885. 



