[NSBCTS WITH INVAGINATED GERM-BAND. 279 



A modification of the type described is met with in Corixa (Metschnikoff, 

 No. 55 ; Brandt, No. 7). In this form the germ-prominence, which becomes 

 invaginated at the posterior pole, is indeed also at first surrounded by food-yolk, 

 but very soon becomes closely applied to the dorsal side of the egg, so that 

 the serosa and the amnion are here in close contact. The germ is consequently 

 not immersed, but superficial. In other respects, the process of rotation and 

 the acquisition of its definitive position by the embryo occur in exactly the 

 same way as in the Libellulidae. 



The germ-bands of the Pediculina and the Mallophaga also, according to 

 Melnikoff (No. 53), agree with regard to position with that of the Libclhilidae. 

 But the condition in these cases is to some extent simpler, as the aperture of 

 invagination into the amniotic cavity remains permanently open. An invaginated 

 germ-band is also found in the Pliysapoda (Dohun, No. 21 ; Jordan, No. 44). 



The processes of development found in the eggs of the Phytophthires form 

 a direct sequence to those described for the Libellulidae. The descriptions 

 given by Metschnikoff (No. 55) and Brandt (No. 7) of the development 

 of the viviparous Coccidae (Aspidiotus, Lecanium) show almost complete agree- 

 ment with the Libellulidae ; and the Psyllidae also, according to Metschnikoff, 

 seem to follow the same course. Certain peculiarities, on the other hand, are 

 shown by the summer eggs of the viviparous Aphidae, which pass through their 

 development within the egg - follicle. The eggs of these forms that develop 

 parthenogenetically do not, as Will (No. 97) has pointed out, undergo the full 

 process of maturation found in other insect eggs. The former exhibit a pre- 

 cocious embiyonic development, which commences at the stage when only the 

 very first phenomena of maturation are to be found in the appearance of small 

 drops of deutoplasm. After the development of the blastoderm, the egg contains 

 only a small amount of food-yolk, which soon disappears (primary food-yolk, Fig. 

 139 A, do), and in which single yolk-cells are found. In the stages that follow, 

 however, the embryo is provided with a fresh mass of yolk (secondary yolk, 

 pseudovitellus, sd) through the development of a kind of placental formation from 

 the follicular epithelium of the parent (Fig. 139 A, sd). At the posterior pole of 

 the embryo, at which the formation of the blastoderm is not fully completed, and 

 where consequently there is a gap in the blastoderm,* a fusion occurs with the 

 corresponding part (x) of the follicular epithelium (/). A mass of cells here 

 develops by repeated division as an outgrowth of the follicular epithelium. This 

 mass, by the degeneration and complete disintegration of the cells composing it, 

 becomes transformed into an accumulation of yolk-spherules (secondary yolk), 

 and the yolk -material thus produced is projected into the interior of the 

 embryo through the gap in the blastoderm (Fig. 139 A, sd). The secondary 

 yolk-mass, which thus comes to lie in the primary body-cavity, and into which 

 yolk-cells (dz) soon wander from the embryo, remains for some time connected 

 by means of a strand of yolk with that part of the follicular epithelium from 

 which it originated. 



The development of the germ -band takes place in the Aphidae by an 

 invagination at the posterior pole in exactly the same way as in the Libellu- 

 lidae. This invagination develops round the gap in the blastoderm already 

 mentioned (Fig. 139 A). It is consequently not closed at its inner end, this 

 being the aperture through which the secondary yolk enters into the interior 



* This gap has been called the blastopore by "Will, and the immigration of 

 yolk-cells proceeding from this point has been assumed to be gastrulation, a 

 view which we cannot share. 



