Development of the Pedipalpi. 423 



function. This structure is likewise found in Phrt/nus, only 

 in this case it does not change its original position, but 

 remains at the base of the second limb. 



When the extremities have already attained a considerable 

 length, and before all the other organs which have just been 

 mentioned have arrived at tlie stage of development which 

 has been described, there begins to be noticeable upon the 

 ventral surface a slight groove, which gradually becomes 

 deeper and finally leads to a sepai*ation between the cephalo- 

 thorax and the abdomen. The former at last bends so far 

 towards the abdomen that the extremities lie close to the sides 

 of the latter. At this period there also takes place the closure 

 of the dorsum. Shortly after the formation of the rudiments 

 of the appendages there are already to be observed on the 

 outer side of the two halves of the primitive streak small 

 quadrate areas which adjoin one another, and in the course of 

 tlie development grow out laterally, to subsequently unite in 

 the median line of the dorsum with the formation of the 

 heart. This closure takes place very rapidly, though we are 

 able to observe that it proceeds from in front towards the 

 rear. It is only at this stage, when the dorsal sides have 

 grown together, that the final development of the ventral 

 surface likewise takes place. Hitherto the six pairs of 

 thoracic ganglia lay, in accordance with their origin, at the 

 sides of the Avidely divergent halves of the primitive streak, 

 separated from one another in the shape of a bifurcate fork, 

 open posteriorly; and in the same way until now the two 

 abdominal ganglionic cords had preserved their position 

 unchanged at the boundary between the dorsal and ventral 

 surfaces. These also now travel towards the median ventral 

 line of the abdomen, and there unite to form a continuous 

 chain. 



With this the development of the embryo is essentially 

 complete. It only remains to be mentioned that some time 

 previously a delicate cuticular envelope was formed, wliich 

 clothes all parts of the body ; upon tiiis envelope we observe 

 at the base of each of the ambulatory limbs, as well as of tlie 

 pedipalpi and chelicerce, a pointed thorn of chitin, which is 

 destined to facilitate the process of hatching. With tiie 

 help of these egg-teeth the embryo now breaks through the 

 shell, stripping off its first cuticular coat in so doing, and 

 attaches itself firmly to the dorsal or ventral surface of the 

 mother, by which it is still carried about for some time 

 longer. 



The just-hatched young at first exhibits only very slow 

 awkward movements, and is so remarkably different from tlie 



