32 ARACHNIDA. 



variations in their method of development appear comparatively unimportant. 

 Attempts have been made to connect the Pseudoscorpiones with other divisions 

 of the Arachnida, especially with the Opiliones, but these have not been 

 sufficiently based on the organisation of the two groups. "We must therefore, 

 according to a recent investigator of the anatomy of the Chernetidae 

 (Cronebekg), leave the systematic position of the Pseudoscorpiones undecided, 

 since their ontogeny, so far as it is yet known, throws no light upon the subject. 



IV. Opiliones. 



The spherical eggs of the Opiliones are surrounded by two 

 membranes. The inner membrane is secreted by the egg, the outer 

 by the epithelium of the genital duct ; they represent the vitelline 

 membrane and the chorion. The eggs, glued together so as to form 

 a large ball, are deposited in a hole in the ground (Henking). The 

 first ontogenetic processes have been closely studied in Opilio and 

 Leiobunum, by Henkixg, but we are unable to accept his view of 

 the origin of the cleavage-nuclei through free nuclear formation, 

 since it contradicts what is known of other Arthropoda.* Accord- 

 ing to Faussek, the egg of Phalangium divides up into a number 

 of large spherical cells filled with yolk-spherules, each cell contain- 

 in" a central nucleus. Cleavage is therefore total. These cells nri«ht 

 be compared to the yolk-pyramids in the eggs of the Araneae, but, in 

 the subsequent processes, these cells in the Opiliones seem to differ 

 from those structures. A cleavage-cavity does not appear. The 

 formation of the blastoderm occurs by the separation and more rapid 

 division of some of the peripheral cells. Xot all the cells, indeed, 

 not even the majority of them, rise to the surface to form the 

 blastoderm, a large proportion of them remain within the egg as 

 yolk-cells (Henkixg, Faussek). The formation of the blastoderm 

 takes place more rapidly in one half of the egg than in the other, 

 a condition similar to that observed in the Araneae. 



Active increase in number of the blastomeres in one region of the 

 blastoderm leads to the formation of a thickening in it; this is 

 the germ-disc. According to Faussek, immigration of cells into the 

 yolk-mass from the disc does not take place ; the entoderm being 

 possibly represented by the cells which remain in the yolk, and 

 from them, at a later stage, the epithelium of the enteron arises. 



The origin of the entoderm from cells which, from the first, remain behind in 

 the yolk, has been assumed for the Araneae (Schimkewitsch), but the forma- 

 tion of the germ-layers in the Opiliones has not yet been observed sufficiently 



* [Most cytologists do not believe in the existence of the process termed 

 free nuclear formation ; all modern research tends to prove that every nucleus 

 has originated directly from a pre-existing one.— Ed.] 



