ox THE DEVELOPMENT OF ARANETXA. 81 



The mesodermic cells of the coxal gland, which was formed in 

 the preceeding stage, are very much differentiaied from the ecto- 

 dermic cells of it. They are the glandulär cells, their size becoming- 

 large and their protoplasm granular and unstainal)le (PI. XV, tig. 

 37). The ectodermic cells form the duct. 



At the distal end of the chelicera^ a solid growth inward of 

 ectodermi<* cells takes place. These cells are surrounded by meso- 

 d(H'mic cells. The distal half of the former becomes the s^landular 

 portion, and its proximal half the duct, of the poison gland, while the 

 mesodermic cells torm the muscular wall of the gland (hg. 39). 



In this stage four paii'ed transverse septa are formed between the 

 four appendage-bearing segments of the abdomen by the sinking 

 of the mesoderm into the yolk. A median unpaired se|)tum, similarly 

 formed, also stretches forward fro]n the posterior end. These septa 

 are foi'med after the disappearance of the cœlomic cavités in the 

 al)domen. In fi,<r. 34, PI, XV., two anterior septa are represented. 

 The first pair of septa probably give rise to the generative organ, 

 and all or some of the others to the so-called liver. 



After undergoing one or two moults, the embryo hatches. The 

 body of the embryo is covered with cuticular hairs. At the end of 

 the pedepalpi and the foui* ambulatory appendages, the claws are pro- 

 duced, and at the end of the chelicerœ the poison fangs, by thickenings 

 of the cuticula. 



Summary. 



(1) The polygonal areas are on the periplasm, and are pro- 

 bably formed when the eggs pass through the oviduct. 



(2) In the process of segmentation the yolk and the nucleus 

 are divided at the same time. The segmentation is syncytial. 



