56 Dr. O. Zacharias on the Development 



" hill," and behind it rises the contracted yelk, and also 

 appears like a hill. This therefore explains how Metschnikoff 

 came to the notion of a germinal and vitelline hill. But such 

 a notion is not justified by the facts, and still less that of a 

 special genital hill from which the reproductive organs are to 

 originate. It is moreover quite incomprehensible that so 

 practised an observer as Metschnikoff even then was could 

 overlook the fact that the tongue of the germinal streak 

 growing freely into the cavity of the blastoderm immediately 

 shows a deep groove in its median line, becomes rounded off 

 on both sides throughout its whole length, and thus produces 

 two distinctly marked germinal pads. Metschnikoff, on the 

 contrary, repeatedly remarks that the embryo in its early 

 stage shows 710 trace of such pads *. 



This negative judgment I can only explain by the fact that 

 Metschnikoff apparently does not practise the " method of 

 rolling," and therefore did not get to see the different aspects 

 of the embryo. In this opinion I am only strengthened by 

 the examination of his figures 16, 17, 18, 19, and 20 (pis. 

 xxviii. and xxix.), as also by the reading of the text relating 

 to them (pp. 444-448). Every one who comes fresh and 

 unprejudiced to the investigation of the development of the 

 Aphides will make the surprising observation that the deve- 

 lopmental processes which finally lead to the formation of the 

 S-shaped germinal streak are not performed in the plane in 

 which Metschnikoff places them in his figures, but in one 

 standing directly perpendicular to it. In my memoir I shall 

 produce the exact proof of this, and also furnish the requisite 

 figures which I have found to be verified in hundreds of 

 preparations. 



As regards the S-shaped germinal streak, which is well 

 known to all investigators of Aphides, the inferior curve of 

 this letter (which is turned to the left) represents the cephalic 

 hood, the same structure, it may be said in passing, which 

 Huxley, entirely mistaking the relative position of the Aphis- 

 germ, characterized as the " abdominal hood." The upper curve 

 (turned to the right) represents the rudiment of the abdomen, 

 and the intermediate part contains the material for the forma- 

 tion of the head and thorax. 



'J'he limhs take their origin from a special superficial layer, 

 the so-called limh-plate (" Extremitatenplatte "), as to the 

 origin of which Metschnikoff has not got at the truth eitlier 

 in Simulia and Corixa or in Aphis Rosa; (see his paper, loc. 

 cit. pp. 400, 427, and 448). For the Aphides I have 



* Zeitschr. f. wiss. Zool. Bd, xvi. (1806), pp. 44b aud 450. 



