44 ON THE ORIGIN AND [CHAP. 



have approached to completion. Indeed, when the 

 legs first appear, they are merely short projections, 

 which it is not always easy to distinguish from the 

 segments themselves. It must, however, be admitted, 

 that the observations are neither so numerous, nor 

 in most cases so full, as could be wished. 



Fig. 30 represents an egg of a May-fly (Phry- 

 ganea), as represented by Zaddach in his excellent 

 memoir, 1 just before the appearance of the appen- 

 dages. It will be seen that a great part of the 

 yolk is still undifferentiated, that the side walls are 

 incomplete, the back quite open, and the segments 

 merely indicated by undulations. This stage is 

 rapidly passed through, and Zaddach only once met 

 with an egg in this condition ; in every other speci- 

 men which had indications of segments, the rudi- 

 ments of the legs had also made their appearance, 

 as in Fig. 31, which, however, as will be seen, does 

 not in other respects show much advance on Fig. 30. 



Again in Aphis, the embryology of which has been 

 so well worked out by Huxley, 2 the case is very 

 similar, although the legs are somewhat later in 

 making their appearance. When the young was ^th 

 of an inch in length, he found the cephalic portion 

 of the embryo beginning, he says, "to extend up- 

 wards again over the anterior face of the germ, so 

 as to constitute its anterior and a small part of its 

 superior wall. This portion is divided by a median 

 fissure into two lobes, which play an important part 



1 Untersuchungen liber die Entwickelung und den Ban der Glieder- 

 thiere, 1854. 



2 Linnean Transactions, vol. xxii. 1858. 



