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



In this connection I would mention that Metschinikoff* figures ccelomic 

 sacs in Simulia exactly similar to those I claim to have discovered in the 

 Fly embryo, although they are unlettered in his figures, and are not referred 

 to in his text. As Metschinikoff only saw them in optical section, he 

 apparently mistook them for the remains of the yelk. Similar appearances 

 are also seen in the embryo of Chironomus. 



4. THE DORSAL ORGAN OF KOWALEVSKI. 



Kowalevski [97] first described a structure in Hydrophilus 

 which he termed the dorsal organ. He regarded it as a 

 thickening of the outer provisional membrane, the serosa, and 

 states that it afterwards forms a tube and sinks into the yelk. 

 Grabert has more recently investigated the subject, and 

 accords with Kowalevski ; but both Graber and Kowalevski 

 regard the whole of the blastoderm beyond the primitive band 

 as serosa. In Lina, Graber derives the dorsal organ from the 

 amnion, and Cholodkowsky states that it is formed from the 

 serosa in Blatta4 



It appears to me that these differences arise partly from 

 a confusion of nomenclature, and partly from the very extra- 

 ordinary changes of position which the amnion is believed to 

 undergo by Graber. The dorsal plate always originates from 

 a portion of the blastocele which is in contact with the yelk. 



Cholodkowsky says that its development in Blatta Germanica 

 is well seen, and that the dorsal margin of the posterior amnial 

 fold ends in a thick plate of contracted serosa, consisting of 

 tolerably high cylindrical cells, the dorsal organ. This plate 

 becomes shortened, and forms an oval fossa, which is in- 

 vaginated in the food-yelk. From the bottom of the invagina- 

 tion large, irregular, often amoeboid, cells arise and wander into 

 the yelk. 



Cholodkowsky's figures convince me that his dorsal organ is 

 my mesenteron. The scattering of the cells in the yelk, which 



* Metschinikoff, E., ' Embryologische Studien an Insecten,' Zeitsch. f. w. 

 Zool., Bd. xvi., 1866. 



f Graber, V., ' Vergleichende Studien iiber die Keimhiillen und die 

 Ruckenbildung der Insecten,' Denkschrift. K. Akad. Math. Naturwiss. Cl. 

 Wien, Bd. lv., 1888. 



% Mem. de 1'Acad. St. Petersbourg, torn, xxxviii. 



