THE DEVELOPMENT OF ADMETUS PUMILIO, 016 



begun to make tlieir appearance. Being unpigmented tliey are 

 not visible on surface views. The lateral eyes are first met 

 with in more mature stages. Just above the chelicerie, on the 

 front of the cephalothorax, we meet on each side with a lappet- 

 like excrescence, the bases of which have already begun to 

 unite with each other where they meet in the median plane; 

 thus they form a kind of mask in front of the embryo^ 

 covering its mouth. The space beneath this mask remains 

 hollow till much later. 



These folds are the Anlagen of the median eyes ; they con- 

 sist of two simple thickenings of the ectoderm^ which have 

 probably been inverted in the way described by Brauer for 

 Scorpions. 



The front wall of the eyes still only consists of a single row 

 of cells ; in the posterior wall the cells have begun to 

 multiply. The space between the anterior and posterior 

 walls of the eyes contains no nuclei, being only filled with 

 cell- walls and plasm. 



The eyes measure in length "1 mm. ; they are "04 mm. 

 thick. 



Besides the eyes there is only one other set of sense- 

 organs to be found. These are the segmental sense-organs 

 first described by Patten (34) as occurring at the base of the 

 legs of Spiders; they were also described by Brauer (7), 

 who found them in Scorpions. The description and drawings 

 given by Brauer accord with those which I could give for 

 Admetus. 



(G) The Alimentary Canal. — The alimentary canal is 

 at this stage still very incomplete, only the most anterior part 

 existing as yet. It consists of a simple tube which just 

 pierces the brain. Its outer anterior end does not pro- 

 ject beyond the brain, the posterior end doing so for 

 about one third of its length. From the point where it 

 leaves the brain it is slightly bent in a ventral direction. 

 The cells forming the walls of the alimentary canal form 

 a thick layer at both ends of the tube. No muscles are 

 as yet observed in connection with the alimentary canal. 



