222 M. Julius Wagner on the 



regartls the region where the yolk-cells are differentiated, no 

 regularity whatever can be detected ; they are formed over 

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

 certain of those 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 ajipearance of the appendages mespderm 

 cells become visible at the sides of the above-mentioned 

 endodermal cluster. From this we see that although this 

 group of cells, which in its position corresponds with the 

 cinnttJus pri7ni'tivus 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 l)y 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. Those doprossious appear to me to 

 correspond to the lateral margins of the germinal streak of 

 Insects (Ileider, IlydrojJdhis). 



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 jjresent a homogeneous structure ; on the contrary, it is 

 difterentiated into sc])arate 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), si)lit8 

 up according to the number of those into separate jjortions, 

 which are still preserved in the intestine of the larva shortly 

 before its escape from the Qg^. 



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

 ]\lite.'!, like tliat of the Spiders, at the stage of the appearance 

 oi the a]ipendages takes up the greater portion of the circum- 

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



