66 WHEELER. [Vol. VIII. 



which necessitate the production of a great number of ova at 

 the expense of their size. The opposite exception occurs in 

 the parasitic Pupipara, where the nourishment of the single 

 larva within the parent is equivalent to the production of a large 

 yolk-laden ^g^y 



The question naturally arises : Were the eggs of the prim- 

 itive Insecta poor or rich in yolk } As all the evidence of com- 

 parative anatomy, embryology and paleontology goes to show 

 that the Metabola are the more recent, the Ametabola the 

 more ancient forms, we are justified in maintaining that prim- 

 itive insects, or at any rate the primitive Pterygota supplied 

 their eggs with a considerable quantity of yolk. At first sight 

 the Apterygota, which have holoblastic eggs, would seem to 

 constitute a serious obstacle to this view, but it must be 

 remembered that total cleavage is not necessarily a criterion of 

 paucity of yolk (witness Arachnida, Crustacea, and Myriopoda). 

 Furthermore, the eggs of some Thysanura, Anurida, e.g. are 

 provided with an abundance of yolk. Holoblastic cleavage in 

 this group is probably a Myriopod trait, as was long ago sug- 

 gested by Metschnikoff ('74). We might perhaps conclude 

 that the superficial type of cleavage, like the embryonic en- 



1 The differences between the eggs of different insects with respect to the 

 amount of yolk is systematically disregarded by Graber ('90). This is shown by 

 his classification of germ-bands as microblastic and macroblastic, brachyblastic and 

 tanyblastic. These distinctions are readily shown to be distinctions in the amount 

 of yolk and not in the germ-band. Thus the just-established germ-bands of the 

 Saltatory Orthoptera appear to be very small because the eggs contain an enor- 

 mous quantity of yolk; while the germ-band of the Muscidas appears correspond- 

 ingly large on account of the small quantity of yolk. The amount of yolk 

 fluctuates even within the limits of the single orders so that the newly-formed germ- 

 bands appear to differ in length more than they really do. In the Orthoptera we 

 have the following series in which the amount of yolk decreases, the germ-band in 

 consequence appearing to increase: Mclanoplus, I\Iantis, CEcanthus, G^ylhis, 

 Xiphidium, Blatta, {?) Gryllotalpa. 



Graber's further classification of germ-bands as orthoblastic and ankyloblastic, 

 or straight and curved, is equally artificial In the great majority of cases the 

 shape of the germ-band depends upon the yolk surface on which it arises, or over 

 which it happens to grow. The uselessness of such a classification is also shown 

 in the case of Xiphidhtm and Orchelimum, where the just-established germ-band 

 is straight, but becomes curved in passing to the dorsal surface, and thereupon 

 again becomes straight. To which of Graber's classes does this germ-band 

 belong ? 



