Dr. Burnett on the Development of Viviparous Aphides. 85 



here commences with the closing up of the abdominal cavity, 

 and the same processes which we have just described are again 

 repeated. 



The details of the development subsequent to this point are 

 like those of the development of ordinary insects or of the Ar- 

 ticulata in general ; and although this ovoid germ has at no time 

 the structural peculiarities of a true ovum — such as a real vitellus, 

 a germinative vesicle and germinative dot ; yet, if we allow a 

 little latitude in our comparison, and regard the vitellus-looking 

 mass as the mucous, and the germ-mass proper as the serous fold 

 of the germinating tissue, as in true eggs — if, I repeat, we can 

 admit this comparison of parts, then the analogy of development 

 between these germs and true eggs of insects may be traced in 

 considerable detail. 



This comparison I have been inclined to admit at least in part, 

 from the striking resemblance of these developing forms at cer- 

 tain stages, with the embryological forms of spiders as they have 

 been figured by Herold* and as I have myself traced them. 

 When, in spiders, the serous fold of the germinating tissue has 

 extended so as to cover two-thirds of the developing form, leaving 

 the vitelline mass on the dorsal surface near one of the poles, 

 the whole embryo quite resembles that of a developing Aphis 

 just before the arches of the segments close up on the back. 



With this view of the relative parts of the germ, the following 

 would be the details of the development of the different systems, 

 and in the noticing of which I shall follow Kollikerf- 



1. The germinating tissue consists of two parts ; a serous and 

 mucous fold. 



2. The abdominal plates arise from the serous fold, sprout out 

 towards the vitelloid mass, pass over it and unite on the dorsal 

 surface of the future animal ; on the opposite side are formed 

 plates which do not unite, but are formed into the hind legs. 



3. The wings are the lateral limbs. 



4. The first traces of the abdominal column appear in the 

 chain of abdominal muscles situated between the nerves and the 

 intestinal canal. 



5. The nervous system in all its parts arises from the serous 

 fold, as well also as the organs of sense. 



6. The mucous fold, or the vitellus-looking mass, serves no pur- 

 pose in the formation until the closing in of the visceral plates. 



7. Thus enclosed in the abdominal cavity, it is not transformed 



* De Generatione Aranearum in ovo. Marbourg, 1824. 



f Observationes de prima Insectorum genesi adjecta Articulatorum evo- 

 lutionis cum Vertebratorum eomparatione. Diss. Inaug. Scr. Alb. Kolliker. 

 Turin, 1812. A work replete with facts and interesting suggestions. 



