of the Viviparous Aphides. 57 



succeeded in proving that the so-called " Extremitatenschicht " 

 is a part of the blastoderm which at a very early period enters 

 into an intimate fusion with the true germinal streak. The 

 details of this are treated of in my memoir. 



Besides the two vertex-plates (Huxley's " procephalic 

 lobes ") which originate from the primitive lateral plates, we 

 have also to distinguish in Aphis-embryos a median plate^ 

 which is produced from the ventral part of the cephalic hood. 

 I would name it the mandibular plate^ as the two mandibles 

 are formed from it. 



The first and second maxillce originate from that part of 

 the limb-plate which overlies the procephalic portion of the 

 germinal streak, or is amalgamated with the latter. The 

 arrangement of the three pairs of buccal organs is of such a 

 kind that a hexagon is formed, the outer angles of which are 

 constituted by the first maxilla. 



Later on the rudiments of the mandibles and first maxillse 

 are enclosed in the depth of the head, and from them originate 

 the " retort-shaped bodies " (of Metschnikoff), which secrete 

 the chitinous stylets of the rostrum. In the mature embryo 

 we perceive two such bodies on each side^ not one only, as 

 Metschnikoff' s figures show. By the demonstration that it 

 really is by the transformation of the mandibles and maxillce 

 that the retort- shaped bodies are produced^ the parts of the 

 mouth of the Aphides are first brought into homology with the 

 corresponding organs in other insects. 



According to Metschnikoff, as is well known, the 

 mandibles and first maxillae are completely retrograded, and 

 the ^' bodies " secreting the piercing seta3 originate quite 

 newly. This was a priori very improbable, and observation 

 gives quite another result. By carefully crushing half- 

 mature embryos the actual conditions may often be very 

 beautifully brought into view. 



Witlaczil, who has treated of the anatomy of the Aphides 

 in much detail in a recent memoir *, gives a very full descrip- 

 tion of the structure of the retort-shaped bodies as studied by 

 him by means of sagittal and transverse sections through fully- 

 developed animals. 



For orientation in many difficult points of the development 

 of Aphides, e. g. as to the question whether or not the Mal- 

 pighian vessels are present in these animals at any time, I 

 have turned to the allied group of the Coccidge, and not 

 without success. Thus I distinctly saw in Coccus hesperidum 

 that the brown masses of substance corresponding to the 

 secondary vitellus in Aphis Rosce arrange themselves very 



* ' Zur Anatomic der Aphiden ' (Wien, 1882). 



