222 M. Julius Wagner on the 



regards the region wliere the yolk-cells are differentiated, no 

 regularity whatever can be detected ; they are formed over 

 the entire surface of the ovum. At the same time, however, 

 certain of these cells form a cluster on the dorsal side of the 

 ovum nearer the posterior end. They appear not to partici- 

 pate in the formation of the yolk-cells, but represent the 

 remaining portion of the endoderm. 



3. Formation of the Mesoderm. — At the stage which imme- 

 diately precedes the appearance of the appendages mesoderm 

 cells become visible at the sides of the above-mentioned 

 endodermal cluster. Fi-om this we see that although this 

 group of cells, which in its position corresponds with the 

 cumulus primitivus of the spiders, does not represent the sole 

 spot where the endoderm cells are formed, on the other hand 

 it is just here that the mesoderm cells arise, which extend 

 hence over the posterior end of the ovum on to the ventral 

 surface, and along this towards the front. As regards the 

 nature of the origin of the mesoderm cells, I find by a careful 

 examination of the sections that at this spot no division in a 

 tangential direction takes place ; that is to say, that we here 

 get an immigration of cells. On each side of the cluster of 

 endodermal cells we find upon the surface of the ovum in the 

 early stages of the development of the mesoderm a furrow- 

 like depression, at the bottom of which an active immigration 

 of the cells takes place. These depressions appear to me to 

 correspond to the lateral margins of the germinal streak of 

 Insects (Heider, Hydrophilus). 



4. Segmentation of the Yolk. — As I have already indicated, 

 no division of the yolk is observable during the segmentation 

 of the ovum. Nevertheless when fresh ova are examined the 

 yolk in the stages of the development of the appendages does 

 not present a homogeneous structure ; on the contrary, it is 

 differentiated into separate polygonal portions of large size, 

 which are divided from one another by clefts. In later stages, 

 when the appendages attain a considerable length, this appear- 

 ance stands out with especial distinctness. I was able to 

 detect it later on in sections also. Thus we see that the yolk, 

 subsequently to the migration of the yolk-cells, which are 

 present at first in inconsiderable numbers (about thirty), splits 

 up according to the number of these into separate portions, 

 which are still preserved in the intestine of the larva shortly 

 before its escape from the egg. 



5. Development of the Legs. — The germinal streak of the 

 Mites, like that of the Spiders, at the stage of the appearance 

 el the appendages takes up the greater portion of the circum- 

 ference of the ovum, passing over on to the dorsal surface 



