THE DORSAL ORGAN OF KOWALEVSKI 255 



he believes occurs, is, I think, the result of imperfect fixation ; 

 and I have no faith in the method employed by Cholodkowsky 

 (chromo-nitric acid). 



Graber gives beautiful figures of the dorsal organ in Hydro- 

 philus, Melolontha, Pyrrhocoris, and Musca, although in the 

 latter he mistook it for a ventral invagination. In all these 

 insects it becomes an epithelial tube. 



Graber, after describing the formation of the dorsal tube by 

 the invagination of the dorsal plate (his dorsal ptychoblast) 

 and its sinking into the yelk, says : ' The cells give off amoe- 

 boid processes, which probably feed upon the yelk.' At a 

 later stage, according to Graber, it separates into its con- 

 stituent cells, which become wander-cells in the yelk. In 

 these statements Graber and Cholodkowsky agree so far as 

 Hydrophilus and Blatta are concerned ; but Graber has more 

 recently described the same organ in the Blow-fly embryo as 

 the rudiment of the hind gut. 



The identity of Graber's hind gut in the fly embryo with 

 Kowalevski's dorsal organ and with my archenteron is certain. 

 The only questions which remain uncertain are : Are Graber 

 and myself right in maintaining that it is part of the alimen- 

 tary canal in the Muscidse ? And is Graber right in regarding 

 it as the hind gut, or am I right in regarding it as a typical 

 gastrulation forming the archenteron ? 



I hold that the dorsal plate, when it first appears in the fly 

 embryo, is neither covered by the amnion nor serosa, and that 

 these membranes extend over it subsequently. Compare 

 Graber's diagram {I.e., p. 254; Fig. 20, p. 18), which shows the 

 membranous fold of the amnion and serosa overlapping it in 

 Hydrophilus. Of course, if the whole blastoderm outside the 

 primitive band is called serosa, and not only its folds (Falten- 

 blatter), then the dorsal plate is developed from the serosa. 



Graber, in Lina, derives it from the amnion, which he thinks 

 first splits in the ventral median line, then travels through the 

 yelk to the dorsum, where its torn edges reunite. This theory 

 appears to me so improbable that I cannot entertain it. 



The idea that the membranes, amnion and serosa, undergo 



