Embryology of the Mites. 223 



with its caudal and cephalic lobes. At this stage it already 

 consists of two cctodeiinal ridges, which arc separated by .1 

 band of flat cells and meet together at the ends of the ellip- 

 tical ovum. When examined from the side the appendages 

 appear very sharply defined. In addition to the usual three 

 pairs of legs, which are characteristic of the larvae of most of 

 the Acarina and especially of Ixodid», the fourth and last 

 pair is also developed in the embryo *. In the course of the 

 development of the anterior legs this pair elongates and even 

 exhibits an indistinct segmentation. Shortly before the larva 

 is hatched it begins to degenerate rapidly ; it decreases in 

 length and becomes, so to speak, drawn in, so that soon no 

 trace of it is left on the outside. But sections of the larva 

 after its escape from the cg^ show that beneath the integu- 

 ment of the body on both sides, in the region of the transition 

 of the mid-gut into the hind-gut (the excretory vesicle), there 

 lies a cluster of cells, which represents the remains of the 

 fourth pair of legs. From these clusters of cells also the 

 fourth pair of appendages appears to develop on the transition 

 of the larva into the nymph. 



6. Segmentation of the Abdomen and its Appendages. — In 

 the early stages of development the mesoderm consists of 

 isolated groups of cells corresponding to the appendages ; the 

 mesoderm cells completely clothe the cavity of the appen- 

 dages, but this cavity remains open to the yolk ; the meso- 

 dermal elements are also localized in a similar manner behind 

 the appendages, that is in the abdomen, since the fourth and 

 last pair of legs must be regarded as appendages of the last 

 thoracic segment. Behind the legs lie from five to six meso- 

 dermal groups. I did not determine their number precisely, 

 because the sixth group passes quite gradually into the meso- 

 derm of the caudal lobes. The internal segmentation in the 

 region of the abdomen is consequently expressed through at 

 least five segments. In the first segment, as in that of tiie 

 thorax, the mesoderm lies in a layer ; in the segments next 

 following it forms a closed half-somite. Most developed arc 

 the second, third, and fourth segments, on each of which 

 there is a projection on each side in the shape of a scarcely 

 perceptible tubercle. These tubercles can only be observed 

 in sectional preparations, and although they apparently do 

 not develop into more noticeable protuberances, they yet 

 without doubt represent structures homologous with the 

 abdominal appendages of spiders. 



7. The Cephalic Appendages. — I can positively assert that. 



* (/. \\'iukler, Gama^us, in no. vii. of Claus's ' Arbeiten.' 



